My Problem:

I want to create functions in Excel that can be used in
the exact same way as built-in functions are used.

For example:
to use the AND built-in function all I need to do is to
type the following into a cell:
=AND(TRUE,FALSE) ..... or
=AND(A1,B1), where cell A1 & B1 contains Boolean values.

The following functions do not exist as built-in
functions in Exce: XNOR & NAND. I would like to write
these functions and access them in the exact same manner
as the AND function:
=XNOR(TRUE,FALSE) or XNOR(A1,B1).

Please help