&-operator compute inversion of formal series sum of ap*X^p, where p=(p1,...,pn). The condition of invertible of formal series is a0 not equal 0. If the condition is not fulfilled than error occurs. The numeric argument of this operation response for maximum exponent of inversion. In example,
a^5 for 2 variables, in this case the highest exponent of inversion of
a is (5,5).
signature(e1 = "fseries", e2 = "numeric")