def __sub__(self, other):
pmf = Pmf()<EOL>for v1, p1 in self.Items():<EOL><INDENT>for v2, p2 in other.Items():<EOL><INDENT>pmf.Incr(v1 - v2, p1 * p2)<EOL><DEDENT><DEDENT>return pmf<EOL>
Computes the Pmf of the diff of values drawn from self and other.

        other: another Pmf

        returns: new Pmf