The following items are legal M expression atoms (expratom). An expression atom is a component of an M expression.
Local variables
Global variables
Intrinsic special variables
Intrinsic functions
Extrinsic functions
Extrinsic special variables
Numeric literals
String literals
An expression enclosed in parentheses
Any of the above preceded by a unary operator
In addition, any of these items may be combined with a binary operator and another expression atom.