trivial groups are non-trivial...``

[fieker]

julia> group(q)
<fp group of size 1 on the generators [ f1 ]>
julia> relators(group(q))
1-element Vector{FPGroupElem}:
 f1
julia> z1, z2 = Oscar.GrpCoh.confluent_fp_group(group(q))
julia> z2
Group homomorphism from 
<fp group on the generators [  ]>
to
<fp group of size 1 on the generators [ f1 ]>

julia> preimage(z2, group(q)[1])
ERROR: Error thrown by GAP: Error, Record Element: '<rec>.wholeGroup' must have an assigned value in
  FamilyObj( mapi[1] ) at /home/fieker/.julia/artifacts/1ae5d8f1898d9dcbe8dc3d93f56369c864a8e717/share/gap/lib/ghomfp.gi:218 called from 
CosetTableFpHom( hom ) at /home/fieker/.julia/artifacts/1ae5d8f1898d9dcbe8dc3d93f56369c864a8e717/share/gap/lib/ghomfp.gi:249 called from
ImagesRepresentative( RestrictedInverseGeneralMapping( hom ), elm ) at /home/fieker/.julia/artifacts/1ae5d8f1898d9dcbe8dc3d93f56369c864a8e717/share/gap/lib/ghom.gi:897 called from
<function "PreImagesRepresentative for GHBI and mult.-elm.-with-inverse">( <arguments> )
 called from read-eval loop at *defin*:0

Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] ThrowObserver(depth::Int32)
   @ GAP ~/.julia/dev/GAP/src/GAP.jl:51
 [3] _call_gap_func(func::GAP.GapObj, a1::GAP.GapObj, a2::GAP.GapObj)
   @ GAP ~/.julia/dev/GAP/src/ccalls.jl:270
 [4] call_gap_func_nokw
   @ ~/.julia/dev/GAP/src/ccalls.jl:224 [inlined]
 [5] (::GAP.GapObj)(a1::GAP.GapObj, a2::GAP.GapObj)
   @ GAP ~/.julia/dev/GAP/src/ccalls.jl:235
 [6] haspreimage(f::GAPGroupHomomorphism{FPGroup, FPGroup}, x::FPGroupElem)
   @ Oscar ~/.julia/dev/Oscar/src/Groups/homomorphisms.jl:274
 [7] preimage(f::GAPGroupHomomorphism{FPGroup, FPGroup}, x::FPGroupElem)
   @ Oscar.GrpCoh ~/.julia/dev/Oscar/experimental/GModule/Cohomology.jl:347
 [8] top-level scope
   @ REPL[72]:1

[fieker]

even more entertaining:

julia> one(group(q)) == gens(group(qq))[1]
true
julia> preimage(z2, one(group(qq)))
<identity ...>

[ThomasBreuer]

The message about the missing wholeGroup is due to the fact that mapi[1] is an empty list and thus its FamilyObj does not know about the group. (If the list would contain group elements then the situation would be different.)
The problem seems to be that this ImagesRepresentative method gets called at all.

[thofma]

Is this still an issue? I forgot how to create an example. Please close if this is fixed.

[fingolfin]

I don't know what q is in the original report, so I can't reproduce this precisely. But I think it is fixed:

julia> g = free_group(0)
<free group of rank zero>

julia> f = free_group(1)
<free group on the generators [ f1 ]>

julia> q, epi = quo(f, gens(f))
(<fp group on the generators [ f1 ]>, Group homomorphism from
<free group on the generators [ f1 ]>
to
<fp group on the generators [ f1 ]>)

julia> phi = hom(g, q, [])
Group homomorphism from
<free group of rank zero>
to
<fp group on the generators [ f1 ]>

julia> preimage(phi, gen(q,1))
<identity ...>