def const_potential(f):
if is_const_potential(f): return f<EOL>elif pimms.is_array(f, '<STR_LIT>'): return PotentialConstant(f)<EOL>else: raise ValueError('<STR_LIT>' % f)<EOL>
const_potential(f) yields f if f is a constant potential function; if f is a constant, yields
  a potential function that always yields f; otherwise raises an error.