def part(f, ii):
    '''
    part(u, ii) for constant or constant potential u yields a constant-potential form of u[ii].
    part(f, ii) for potential function f yields a potential function g(x) that is equivalent to
      f(x[ii]).
    '''
    f = to_potential(f)
    if is_const_potential(f): return PotentialConstant(f.c[ii])
    else:                     return compose(PotentialPart(ii), to_potential(f))
part(u, ii) for constant or constant potential u yields a constant-potential form of u[ii].
    part(f, ii) for potential function f yields a potential function g(x) that is equivalent to
      f(x[ii]).