BETA _ _ _ _ _ _ _ _ _ _ _ _ operator
The Beta operator returns the Beta function defined by
Beta (z,w) := defint(t**(z-1)* (1 - t)**(w-1),t,0,1) .
Beta(2,2); 1 / 6 Beta(x,y); gamma(x)*gamma(y) / gamma(x + y)
The operator Beta is simplified towards the GAMMA operator.