Make a selection. Try to decide what it is (ill-formed, a
variable/constant, a combinator, an application).
Hint:
variables/constants start lower case and then can be most anything
the combinators are upper case B C I K S W Y
applications are two combinator expressions separated by a space or @
brackets can be used around combinator expressions
applications left associate, but brackets can be used to get right association
Try to decide how it parses
(eg what is applied to what). Ask the software. Devise your own. [And
if you are ever puzzled later you can come back here and get your
expression parsed.]