TODO.

TODO.md

 ## TO DO
 
+* We probably want a lazy variant of the combinator `**`, at the level
+  of sequences and/or enumerations, so that if the left-hand argument
+  produces an empty sequence, then the right-hand argument is not
+  evaluated.
+
 * Write some documentation.
 
 * Test. Check that `length`, `get`, `foreach` and `to_seq` are consistent.