Unfortunately,

: myexp ( u1 u2 -- u3 ) \ u3 = u1^u2   over swap 1 ?do over * loop nip ;

Hangs if the exponent is 0. The correct answer is 1.

: myexp ( u1 u2 -- u3 ) \ u3 = u1^u2   1 swap 0 ?do over * loop nip ;

Will fix that, but it hangs if the exponent is negative.