About using find with prune option

I have a source folder where you can find files with extensions .c, .pc, .pro.c.

I want to find all files with .c and .pc only, since .pro.c is a generated file from .pc after preprocessing.

Here is my find expression which is not yet correct:

find source ( ( -name '*.pro.c' ) -a -prune ) -o ( -name '*.c' -a -print )

What is my mistake ?

Can someone help me and explain why I do not get the correct result ?

Thank you.

find source -name '*.c' ! -name '*.pro.c' -o -name '*.pc'