Lexical substitution during macro expansion

Lexical substitution is employed to produce an assembly statement from a model statement. A model statement contains 3 types of strings:

a. An ordinary string that stands for itself.

b. The name of a formal parameter that is preceded by the character '&'.

c. The name of a preprocessor variable that is as well preceded by the character '&'.

Throughout lexical expansion, strings of type 1 are retained with no substitution. Strings of sorts 2 and 3 are replaced by the 'values' of the formal parameters or preprocessor variables. The value of a formal parameter is the subsequent to the actual parameter string.