Builtin macros destroy ?a

Using builtin macros cause argument field wildcards in procedures to become undefined if they are named `?a`:

```form
#procedure foo(?a)
  #message `toupper_(abc)'
  #message `?a'
#endprocedure

#call foo(1,2,3)
.end
```
```
~~~ABC
foo Line 3 ==> Undefined preprocessor variable ?a
```

Workaround: Use `?b` etc. instead when defining procedures.