See also @ compose 

Example

f(x)=sin(x)+2

f(x)=sin(x)+2

g = f + identity("x")

g(x) = 2+sin(x)+x

h = g*(-5 + identity("y"))

h(x, y) = -10-5*sin(x)+sin(x)*y-5*x+x*y+2*y

k = identity("x")

k(x) = x

k2 = k*k

k2(x) = x^2