IdentityMapping( D )
IdentityMapping returns the identity mapping on the domain D.
    gap> g := Group( (1,2,3,4), (2,4), (5,6,7) );;  g.name := "g";;
    gap> i := IdentityMapping( g );
    IdentityMapping( g )
    gap> (1,2,3,4) ^ i;
    (1,2,3,4)
    gap> IsBijection( i );
    true 
IdentityMapping  calls   D.operations.IdentityMapping(  D  )  and
returns this value.
The functions usually called  this way  are GroupOps.IdentityMapping if
the domain  D is a group and  FieldOps.IdentityMapping  if the domain
D is a field.
GAP 3.4.4