def erf(f=Ellipsis):
f = to_potential(f)<EOL>if is_const_potential(f): return const_potential(np.erf(f.c))<EOL>elif is_identity_potential(f): return ErfPotential()<EOL>else: return compose(ErfPotential(), f)<EOL>
erf(x) yields a potential function that calculates the error function over the input x. If x is
  a constant, yields a constant potential function.
erf() is equivalent to erf(...), which is just the error function, calculated over its inputs.