This turns a bunch of ifs on their heads.
We often see this pattern in te:
```c
if (s->type != SOME_TYPE) {
// error handling
} else {
// normal code
}
```
Only, since we want to return the first error, we do
```c
if (s->type == SOME_TYPE) {
// normal code
} else if (s->type != TOK_ERROR) {
// Add a new error - if it already has type error
// this should already be handled.
}
```
One big issue is the comma operator, that means arity-1 functions can
take an arbitrary number of arguments. E.g.
```fish
math "sin(5,9)"
```
will return the value of sin for _9_, since this is read as "5 COMMA
9".
