IsFFE( obj )
IsFFE returns true if obj, which may be an object of an arbitrary
type, is an element in a finite field and false otherwise. Will signal
an error if obj is an unbound variable.
Note that integers, even though they can be multiplied with elements in
finite fields, are not considered themselves elements in finite fields.
Therefore IsFFE will return false for integer arguments.
gap> IsFFE( Z(2^4)^7 );
true
gap> IsFFE( 5 );
false
GAP 3.4.4