How to assign values to variables

Hi,

I am using maxima to program a simple physic model.
Let's imagine that at some point of my program, I get a matrix like :

U:MATRIX([U1x],[U1y],[U2x],[U2y])

and an other one like

Fm:MATRIX([SIN(x)],[COS(x)],[SIN(x)^2],[ATAN(x)])

I am looking for a way to assign Fm elements to U elements,
element-per-elements.
I mean that at the end of the process, typing U1y; should return cos(x)
for exemple, or U2x; should return sin(x)^2.

I have been looking for that in many docs, without success.
Thanks in advance.

Paul