Implementation of stream_cons

In order to implement stream_conswe require two other primitives delay and force. does not evaluate but delays the evaluation. force takes a delayed object and evaluates it. Using these two primitives we can define and the associated and functions.

stream_cons(a, b) =  cons(a, delay(b))
hd(s) = car(s)
tl(s) = force(cdr(s))

Note that delay can be easily implemented by wrapping a around the term being delayed. force just uses apply on the delayed object.
delay(b) = )
force(e) = e()
Here stands for unit.