%
% Polynomial power  h = f^k
%
% Syntax:  >> h = PolynomialPower(f,k)
%     or   >> h = ppower(f,k)
%
% Input:     f --- (string)  polynomial
%            k --- (integer) exponent
%
% Output:    h --- (string) polynomial f^k if k is nonnegative
%
% Example:   >> PolynomialPower('x*y+1',3)
%
%            ans =
%
%            1 + 3*x*y + 3*x^2*y^2 + x^3*y^3
