LIST

1. WHY WE NEED LISTS
====================

We need lists because 
	(cons 1 2) 
will immediately tell students that they don't have a list.
If they first need to process the pair with some function, 
the error is discovered way too late and it doesn't make 
sense to them. With RUST they can also see where the erroneous 
"list" is truly created. In case they use pairs, they can only 
see where some function expected a NULL list and that it didn't
get one. Finally, a function may even work just fine on an ill
defined version of a function: 

  reverse(l) = if empty?(l) then {} else reverse(l):first(l)

will immediately be caught as wrong when applied to a list of 
numbers. In Scheme it apparently works and the output apparently
looks like a list :).

2. LIST SYNTAX
==============

Infix notation for cons is natural. Indeed "," is most natural. 
But it is unnatural to say that 
	1,2,3,empty
is a list with 3 elements. So we definitely need a syntax for
long lists that avoids empty: 
	1,2,3.
where . abbreviates ,null. 

Problem 1: what is f(1,2,3) ?
  So we need to use something else than ",". 

Problem 2: how do we print 1,2,3,empty? We print it as 1,2,3. because 
  this is natural. But then 1,2,3,empty should "evaluate" to 1,2,3.
  and how come they look the same? 

This suggests a syntax for "long lists" that is different from the syntax
for short lists AND something other than , as the infix cons. [] are too
natural for matrix notation. {} is the only pairing thing left, unless we
want to write list(...) or matrix[...]. Since lists are not too far away
from sets, we decided on set notation for long lists and colon for infix
cons. 

	1:2:3:empty --> {1,2,3}