Λ-calculus using binary numbers of variable length (lambdaway.fr)

The goal is to build some efficient binary arithmetic at the λ-calculus level, that is to say using a reduced set of lambdatalk functionalities, two special forms, lambda and def, no data or control structures - if then else doesn't exist - and an empty dictionary, at least for the main algorithms. For instance: fibonacci(100) = 354224848179261915075 factorial(22) = 1124000727777607680000

I do not understand this one:

{def PAIR {lambda {:a :b :c} {:c :a :b}}}

What is :c if called with two arguments? And what is its application?