IsCyclotomicField( obj )
returns true if obj is a number field record (see IsNumberField)
where obj.isCyclotomicField = true, and false else.
gap> IsCyclotomicField( CF(9) );
true
gap> IsCyclotomicField( NF( [ ER(-3) ] ) );
true
gap> IsCyclotomicField( NF( [ ER(3) ] ) );
false
GAP 3.4.4