<programming> Any piece of program code in a high-level
language which, when (if) its execution terminates, returns a
value. In most programming languages, expressions consist of
constants, variables, operators, functions, and parentheses.
The operators and functions may be built-in or user defined.
Languages differ on how expressions of different types may
be combined - with some combination of explicit casts and
implicit coercions.
The syntax of expressions generally follows conventional
mathematical notation, though some languages such as Lisp or
Forth have their own idiosyncratic syntax.
