Dear Dima,

Let g:=Group(....). Is there any natural way to avoid calling a procedure
checking whether h:=Subgroup(g,[...]) is indeed a subgroup in the
moment of creating h?

in most cases avoiding the dispatcher will disable all test, eg, in
order to create a subgroup without checking the generators one can use
'h := g.operations.Subgroup( Parent(g), [ <gens> ] )'. However, if
<gens> are not elements of <g> computations with <h> might give wrong
result without warning.

best wishes
Frank