function Y = dirac(X)
%DIRAC Delta function.
% DIRAC(X) is zero for all X, except X == 0 where it is infinite.
% DIRAC(X) is not a function in the strict sense, but rather a
% distribution with int(dirac(x-a)*f(x),-inf,inf) = f(a) and
% diff(heaviside(x),x) =...