Is there an easy way to sub an equation in for a letter in grasshopper?

Say I have an equation that is:

Panel:

1/2*(A^2+B^2+C^2)*(A^2+B^2+C^2)-(A^2+B^2+C^2)-(A^2+B^2+C^2)

Where:

A=sin(x)*sin(y)

B=cos(x)*cos(y)

C=cos(y)

I’d like to be able to do something along the lines of “find and replace” the A and replace it with the sin(x)*sin(y)