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  6 Changes between GAP 4.5 and GAP 4.6
  
  This  chapter  lists  most important changes between GAP 4.5.7 and GAP 4.6.2
  (i.e.  between  the  last release of GAP 4.5 and the first public release of
  GAP  4.6). It also contains information about subsequent update releases for
  GAP 4.6.
  
  
  6.1 GAP 4.6.2 (February 2013)
  
  
  6.1-1 Changes in the core GAP system introduced in GAP 4.6
  
  Improved and extended functionality:
  
      It  is now possible to declare a name as an operation with two or more
        arguments   (possibly  several  times)  and  THEN  declare  it  as  an
        attribute.  Previously this was only possible in the other order. This
        should make the system more independent of the order in which packages
        are loaded.
  
      Words  in fp groups are now printed in factorised form if possible and
        not too time-consuming, i.e. a*b*a*b*a*b will be printed as (a*b)^3.
  
      Added methods to calculate Hall subgroups in nonsolvable groups.
  
      Added  a generic method for IsPSolvable (Reference: IsPSolvable) and a
        better  generic  method for IsPNilpotent (Reference: IsPNilpotent) for
        groups.
  
      Improvements  to  action  homomorphisms:  image  of an element can use
        existing  stabiliser chain of the image group (to reduce the number of
        images   to   compute),   preimages   under  linear/projective  action
        homomorphisms use linear algebra to avoid factorisation.
  
      To improve efficiency, additional code was added to make sure that the
        HomePcgs   of   a   permutation   group   is   in   IsPcgsPermGroupRep
        representation in more cases.
  
      Added  an  operation SortBy (Reference: SortBy) with arguments being a
        function  f  of  one  argument and a list l to be sorted in such a way
        that l(f[i]) <= l(f[i+1]).
  
      Added  a  kernel  function  MEET_BLIST  which  returns true if the two
        boolean  lists  have  true in any common position and false otherwise.
        This  is  useful  for  representing  subsets of a fixed set by boolean
        lists.
  
      When assigning to a position in a compressed FFE vector GAP now checks
        to  see  if the value being assigned can be converted into an internal
        FFE  element  if  it  isn't  one  already.  This  uses  new  attribute
        AsInternalFFE   (Reference:  AsInternalFFE),  for  which  methods  are
        installed for internal FFEs, Conway FFEs and ZmodpZ objects.
  
      Replaced  ViewObj (Reference: ViewObj) method for fields by ViewString
        (Reference: ViewString) method to improve the way how polynomial rings
        over algebraic extenstions of fields are displayed.
  
      Made  the info string (optional 2nd argument to InstallImmediateMethod
        (Reference:  InstallImmediateMethod))  behave  similarly  to  the info
        string  in  InstallMethod  (Reference:  InstallMethod). In particular,
        TraceImmediateMethods  (Reference:  TraceImmediateMethods)  now always
        prints the name of the operation.
  
      Syntax  errors  such  as Unbind(x,1) had the unhelpful property that x
        got unbound before the syntax error was reported. A specific check was
        added to catch this and similar cases a little earlier.
  
      Allow  a GAPARGS parameter to the top-level GAP Makefile to pass extra
        arguments to the GAP used for manual building.
  
      Added      an      attribute     UnderlyingRingElement     (Reference:
        UnderlyingRingElement) for Lie objects.
  
      The  function  PrimeDivisors  (Reference: PrimeDivisors) now became an
        attribute. [suggested by Mohamed Barakat]
  
      Added  an  operation  DistancePerms  (Reference: DistancePerms) with a
        kernel method for internal permutations and a generic method.
  
      Added  a  method for Subfields (Reference: Subfields) to support large
        finite fields. [reported by Inneke van Gelder]
  
  Fixed bugs which could lead to crashes:
  
      The  extremely  old  DEBUG_DEADSONS_BAGS  compile-time  option has not
        worked  correctly  for  many years and indeed crashes GAP. The type of
        bug  it  is  there  to detect has not arisen in many years and we have
        certainly  not  used this option, so it has been removed. [Reported by
        Volker Braun]
  
  Other fixed bugs:
  
      Scanning  of  floating  point  literals  collided with iterated use of
        integers as record field elements in expressions like r.1.2.
  
      Fixed  two  potential  problems in NorSerPermPcgs, one corrupting some
        internal data and one possibly mixing up different pcgs.
  
      Fixed   a   performance   problem  with  NiceMonomorphism  (Reference:
        NiceMonomorphism). [reported by John Bamberg]
  
      Fixed  a  bug  in  ReadCSV  (Reference: ReadCSV) that caused some .csv
        files being parsed incorrectly.
  
  No longer supported:
  
      The   file   lib/consistency.g,  which  contained  three  undocumented
        auxiliary  functions,  has been removed from the library. In addition,
        the  global  record Revision is now deprecated, so there is no need to
        bind its components in GAP packages.
  
  
  6.1-2 New and updated packages since GAP 4.5.4
  
  At  the  time of the release of GAP 4.5 there were 99 packages redistributed
  with GAP. The first public release of GAP 4.6 contains 106 packages.
  
  The  new  packages  that  have  been  added  to the redistribution since the
  release of GAP 4.5.4 are:
  
      AutoDoc   package   by  S.  Gutsche,  providing  tools  for  automated
        generation of GAPDoc manuals.
  
      Congruence  package  by  A.  Konovalov,  which  provides  functions to
        construct  various canonical congruence subgroups in SL_2(ℤ), and also
        intersections  of  a  finite  number of such subgroups, implements the
        algorithm  for  generating  Farey symbols for congruence subgroups and
        uses  it  to  produce  a  system  of  independent generators for these
        subgroups.
  
      Convex package by S. Gutsche, which provides structures and algorithms
        for convex geometry.
  
      Float  package  by  L.  Bartholdi,  which  extends  GAP floating-point
        capabilities    by   providing   new   floating-point   handlers   for
        high-precision real, interval and complex arithmetic using MPFR, MPFI,
        MPC   or   CXSC   external   libraries.   It   also  contains  a  very
        high-performance  implementation  of  the LLL (Lenstra-Lenstra-Lovász)
        lattice reduction algorithm via the external library FPLLL.
  
      PolymakeInterface  package  by T. Baechler and S. Gutsche, providing a
        link    to    the    callable   library   of   the   polymake   system
        (http://www.polymake.org).
  
      ToolsForHomalg   package   by   M.   Barakat,   S.   Gutsche   and  M.
        Lange-Hegermann,  which  provides some auxiliary functionality for the
        homalg project (http://homalg.math.rwth-aachen.de/).
  
      ToricVarieties  package  by S. Gutsche, which provides data structures
        to  handle  toric varieties by their commutative algebra structure and
        by their combinatorics.
  
  Furthermore,  some  packages  have  been  upgraded  substantially  since the
  GAP 4.5.4 release:
  
      Starting  from  2.x.x, the functionality for iterated monodromy groups
        has  been  moved  from  the  FR  package by L. Bartholdi to a separate
        package     IMG     (currently     undeposited,     available     from
        https://github.com/laurentbartholdi/img).  This completely removes the
        dependency  of  FR  on  external  library modules, and should make its
        installation much easier.
  
  
  6.2 GAP 4.6.3 (March 2013)
  
  Improved functionality:
  
      Several  changes were made to IdentityMat (Reference: IdentityMat) and
        NullMat (Reference: NullMat). First off, the documentation was changed
        to  properly  state  that these functions support arbitrary rings, and
        not just fields. Also, more usage examples were added to the manual.
  
        For  NullMat, it is now also always possible to specify a ring element
        instead  of  a  ring,  and  this  is documented. This matches existing
        IdentityMat  behavior, and partially worked before (undocumented), but
        in some cases could run into error or infinite recursion.
  
        In  the  other  direction,  if a finite field element, IdentityMat now
        really  creates  a  matrix  over  the  smallest  field containing that
        element.  Previously,  a  matrix  over  the  prime  field  was created
        instead, contrary to the documentation.
  
        Furthermore, IdentityMat over small finite fields is now substantially
        faster  when  creating  matrices of large dimension (say a thousand or
        so).
  
        Finally,   MutableIdentityMat   (Reference:   MutableIdentityMat)  and
        MutableNullMat  (Reference:  MutableNullMat)  were explicitly declared
        obsolete  (and  may  be  removed  in  GAP  4.7).  They  actually  were
        deprecated  since  GAP  4.1,  and their use discouraged by the manual.
        Code  using them should switch to IdentityMat (Reference: IdentityMat)
        respectively NullMat (Reference: NullMat).
  
      Two  new  PerfectResiduum  (Reference:  PerfectResiduum)  methods were
        added for solvable and perfect groups, handling these cases optimally.
        Moreover,  the  existing generic method was improved by changing it to
        use     DerivedSeriesOfGroup     (Reference:    DerivedSeriesOfGroup).
        Previously,  it  would  always compute the derived series from scratch
        and then throw away the result.
  
      A  new  MinimalGeneratingSet  (Reference: MinimalGeneratingSet) method
        for  groups  handled by a nice monomorphisms was added, similar to the
        existing  SmallGeneratingSet  (Reference:  SmallGeneratingSet) method.
        This  is  useful if the nice monomorphism is already mapping into a pc
        or pcp group.
  
      Added    a    special    method    for   DerivedSubgroup   (Reference:
        DerivedSubgroup) if the group is known to be abelian.
  
  Fixed bugs:
  
      Fixed  a bug in PowerModInt (Reference: PowerModInt) computing r^e mod
        m in a special case when e=0 and m=0. [Reported by Ignat Soroko]
  
      CoefficientsQadic (Reference: CoefficientsQadic) now better checks its
        arguments  to  avoid  an  infinite  loop when being asked for a q-adic
        representation for q=1. [Reported by Ignat Soroko]
  
      Methods    for    SylowSubgroupOp   (see   SylowSubgroup   (Reference:
        SylowSubgroup)) for symmetric and alternating group did not always set
        IsPGroup    (Reference:    IsPGroup)   and   PrimePGroup   (Reference:
        PrimePGroup) for the returned Sylow subgroup.
  
      Display  of  matrices  consisting  of Conway field elements (which are
        displayed as polynomials) did not print constant 1 terms.
  
      Added  an  extra  check  and  a  better error message in the method to
        access  natural  generators  of  domains  using  the  .  operator (see
        GeneratorsOfDomain (Reference: GeneratorsOfDomain)).
  
      Trying  to  solve  the  word  problem in an fp group where one or more
        generators  has  a name of more than one alphabetic character led to a
        break loop.
  
      Provided the default method for AbsoluteIrreducibleModules (Reference:
        AbsoluteIrreducibleModules)  as a temporary workaround for the problem
        which  may  cause  returning  wrong results or producing an error when
        being called for a non-prime field.
  
      A bug in the GAP kernel caused RNamObj to error out when called with a
        string  that had the IsSSortedList (Reference: IsSSortedList) property
        set  (regardless of whether it was set to true or false). This in turn
        would  lead  to  strange  (and inappropriate) errors when using such a
        string to access entries of a record.
  
      GAP can store vectors over small finite fields (size at most 256) in a
        special  internal  data  representation where each entry of the vector
        uses  exactly  one byte. Due to an off-by-one bug, the case of a field
        with  exactly  256 elements was not handled properly. As a result, GAP
        failed  to  convert a vector to the special data representation, which
        in  some  situations  could  lead to a crash. The off-by-one error was
        fixed and now vectors over GF(256) work as expected.
  
      A  bug  in  the  code  for accessing sublist via the list{poss} syntax
        could lead to GAP crashing. Specifically, if the list was a compressed
        vector  over  a finite field, and the sublist syntax was nested, as in
        vec{poss1}{poss2}.  This  now  correctly  triggers an error instead of
        crashing.
  
  New packages added for the redistribution with GAP:
  
      SpinSym  package  by  L.  Maas,  which contains Brauer tables of Schur
        covers  of  symmetric and alternating groups and provides some related
        functionalities.
  
  
  6.3 GAP 4.6.4 (April 2013)
  
  New functionality:
  
      New  command line option -O was introduced to disable loading obsolete
        variables.  This  option  may be used, for example, to check that they
        are  not  used  in  a  GAP  package or one's own GAP code. For further
        details  see  Reference:  options  and Reference: Replaced and Removed
        Command Names.
  
  Fixed bugs which could lead to incorrect results:
  
      Fixed     the     bug     in    NewmanInfinityCriterion    (Reference:
        NewmanInfinityCriterion)  which  may  cause  returning true instead of
        false. [Reported by Lev Glebsky]
  
  Fixed bugs which could lead to crashes:
  
      Fixed  the  kernel method for Remove (Reference: Remove) which did not
        raise  an  error in case of empty lists, but corrupted the object. The
        error  message  in  a  library  method  is also improved. [Reported by
        Roberto Ràdina]
  
  Fixed bugs that could lead to break loops:
  
      Fixed  requirements  in  a  method to multiply a list and an algebraic
        element. [Reported by Sebastian Gutsche]
  
      Fixed  a  bug  in  NaturalCharacter (Reference: NaturalCharacter for a
        group) entering a break loop when being called on a homomorphism whose
        image is not a permutation group. [Reported by Sebastian Gutsche]
  
      Fixed     a     bug     in     ExponentsConjugateLayer     (Reference:
        ExponentsConjugateLayer)  which occured, for example, in some calls of
        SubgroupsSolvableGroup  (Reference:  SubgroupsSolvableGroup) [Reported
        by Ramon Esteban-Romero]
  
      Fixed    a    problem    with   displaying   function   fields,   e.g.
        Field(Indeterminate(Rationals,"x")). [Reported by Jan Willem Knopper]
  
      Fixed  two bugs in the code for NaturalHomomorphismByIdeal (Reference:
        NaturalHomomorphismByIdeal)  for polynomial rings. [Reported by Martin
        Leuner]
  
      Added missing method for String (Reference: String) for -infinity.
  
      Fixed  the  bug  with  ONanScottType  (Reference:  ONanScottType)  not
        recognising product action properly in some cases.
  
      The  method  for  SlotUsagePattern  (Reference:  SlotUsagePattern) for
        straight  line  programs  had  a  bug which triggered an error, if the
        straight line program contained unnecessary steps.
  
  
  6.4 GAP 4.6.5 (July 2013)
  
  Improved functionality:
  
      TraceMethods    (Reference:    TraceMethods    for   operations)   and
        UntraceMethods  (Reference:  UntraceMethods for operations) now better
        check  their  arguments  and provide a sensible error message if being
        called  without arguments. Also, both variants of calling them are now
        documented.
  
      Library  methods  for  Sortex  (Reference: Sortex) are now replaced by
        faster  ones  using  the kernel SortParallel (Reference: SortParallel)
        functionality instead of making expensive zipped lists.
  
  Fixed bugs which could lead to incorrect results:
  
      IntHexString   (Reference:  IntHexString)  wrongly  produced  a  large
        integer  when  there  were  too  many  leading zeros. [Reported by Joe
        Bohanon]
  
  Fixed bugs that could lead to break loops:
  
      A    bug    that    may    occur   in   some   cases   while   calling
        TransitiveIdentification     (transgrp:     TransitiveIdentification).
        [Reported by Izumi Miyamoto]
  
      The new code for semidirect products of permutation groups, introduced
        in  GAP  4.6,  had  a  bug  which  was causing problems for Projection
        (Reference: Projection). [Reported by Graham Ellis]