def gradient(self):
grad = {}<EOL>for i, f in enumerate(self._means):<EOL><INDENT>for varname, g in f.gradient().items():<EOL><INDENT>grad[f"<STR_LIT>"] = g<EOL><DEDENT><DEDENT>return grad<EOL>
Sum of mean function derivatives.

Returns
-------
∂𝐦 : dict
    ∂𝐟₀ + ∂𝐟₁ + ….