Write a denotational semantics mapping function for an if statement.

M_if(if B then L, s) where B is a boolean expression, L is the list of statements, and s is the current program state.

Assume M_b(B,s) is the denotational semantics function that returns the boolean value of B calculated at the program state, and M_L(L,s) is the denotational semantics function that returns the new program state after performing the list of statements L, starting at the program state s.