The following doesn't seem to work:

)set func comp on

f(A:List PI, B:List PI): FRAC INT == cap(reduce(*, map(i +-> elementary i,
A)), reduce(*, map(i+-> complete i, B)))

   Function declaration f : (List(PositiveInteger),List(PositiveInteger
      )) -> Fraction(Integer) has been added to workspace.

Type: Void

f([1,2,3],[2,2,2])

Compiling function f with type (List(PositiveInteger),List(
      PositiveInteger)) -> Fraction(Integer)

(1)

Type: Fraction(Integer)

Replacing map(...) by '[...]?' seems to resolve the problem.

Martin