def part(f, ii):
f = to_potential(f)<EOL>if is_const_potential(f): return PotentialConstant(f.c[ii])<EOL>else:                     return compose(PotentialPart(ii), to_potential(f))<EOL>
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]).