Only 21 lines of code to generate a list with all untyped lambda terms (with repetitions)

A few steps:

1. take a function that, for a list of terms of at most n variables, creates a new list of terms by turning the leaves (vars) of the original list into connectives (appl, abst) in the output list.
2. take a function that iteratively concatenates the outputs of 1, starting from the list of all n variables, these are all terms with at most n variables.
3. note that the list of all final lists for 1, 2, ... variables, contains all terms of all amounts of variables
4. print all terms in the lists from 2. using a diagonal (cantor) traversal, to let all elements of all infinite lists to eventually appear.