String expressions consist of combinations of string literals, string variables, string manipulation operators, and parentheses. See:

Characters that cannot be placed in string literals can be placed in string expressions using the :CHR: unary operator. Any ASCII character from 0 to 255 is permitted.

The value of a string expression cannot exceed 512 characters in length. It can be of zero length.

improb  SETS    "literal":CC:(strvar2:LEFT:4)
            ; sets the variable improb to the value "literal"
            ; with the left-most four characters of the
            ; contents of string variable strvar2 appended