The main function that operates on indenting-printers is emit, which takes the printer and a string and emits the string to the printer’s output stream, keeping track of when it emits a newline so it can reset the beginning-of-line-p slot.

The helper indent-if-necessary checks beginning-of-line-p and indenting-p to determine whether it needs to emit indentation and, if they’re both true, emits as many spaces as indicated by the value of . Code that uses the indenting-printer can control the indentation by manipulating the indentation and indenting-p slots. Incrementing and decrementing indentation changes the number of leading spaces, while setting indenting-p to **NIL** can temporarily turn off indentation.

With those preliminaries out of the way, you’re ready to get to the guts of the FOO processor.