Now, the following example produces a syntax error.
```lean
macro "foo!" x:term : term => `($x + 1)
check id foo! 10
```
@Kha, I think the heuristic is simple and defensible.
If the new syntax starts and ends with token, than the precedence is
`maxPrec`. Otherwise, it is `leadPrec`.
see #180
@Kha Before this commit, we were producing the error "expected command" at the `let` token
```lean
check id let x := 1; x
```
The new error is "expected command, but found term; this error may be
due to parsing precedence levels, consider parenthesizing the term".
The example above looks artificial, but it will happen all the time as
users start to define their own notation.
