Lambda the Ultimate

I am trying to implement heap in CLisp. I have already done it by making a linear data strcuture with no left / right pointers. But now I want to use a nested list format which contains (el () ()) at the highst level.
The problem I am facing with is how to define a recursive case for insertion. Heap requires the tree to be complete.

I approach the issue as follows:

1. I will take a heap and an element
2. I will add the element in the last node keeping it a complete tree
3. I wil swap the child with parent if it is greater than the parent

But I am stuck at the second step i.e. how to add an element without making the tree incomplete? KIndly help me on this!!!!!!!