The assembler analyzes the statements it processes during lookahead, only to establish attributes of symbols in their name fields.

Variable symbols are not replaced. Modifier expressions are evaluated only if all symbols involved were defined prior to lookahead. Possible multiple or inconsistent definition of the same symbol is not diagnosed during lookahead because conditional assembly might eliminate one (or more) of the definitions.

Lookahead does not check undefined operation codes against library macro names. If the name field contains an ordinary symbol and the operation code cannot be matched with one in the current operation code table, then the ordinary symbol is assigned the type attribute of M. If the operation code contains special characters or is a variable symbol, a type attribute of U is assumed. This can be wrong if the undefined operation code is later substituted with a known operation code or is later defined by OPSYN. OPSYN statements are not processed; thus, labels are treated in accordance with the operation code definitions in effect at the time of entry to lookahead.