Make a selection. Try to decide what it is (ill-formed, a variable, an application, an abstraction). 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.]
Some systems use juxtaposition to mean application, so 'ab' means 'a@b'. This is fine except that it requires that variables have length one so that we know that 'ab' is two variables juxtaposed not one variable of length 2. But we want to labels like 'firstVariable' to mean a single variable, so we cannot use this juxtaposition convention.