The conditional assembly instructions AGO and AIF in open code control the sequence in which source statements are processed. Using these instructions it is possible to branch back to a sequence symbol label and reuse previously processed statements. Due to operating system restrictions, the primary input source can only be read sequentially, and cannot be reread. Whenever a sequence symbol in the name field is encountered in open code, the assembler must assume that all subsequent statements might need to be processed more than once. The assembler uses the lookahead file to save the statement containing the sequence symbol label and all subsequent statements as they are read and processed. Any subsequent AGO or AIF to a previously encountered sequence symbol is resolved to an offset into the lookahead file and input continues from that point.