4 parameters -- a, b, c, and d
- a -- leave out of the call
- b -- include, but without an =
- c -- include, with an = but no value
- d -- include, with an = and a value
| name | #if, no bar | #if, with bar | subst |
|---|---|---|---|
| a | non-empty | empty | subst |
| b | non-empty | empty | subst |
| c | non-empty | empty | subst |
| d | non-empty | empty | subst |
