location notification for unequal left and right element count

[AdamantConlanger]

"found 2 elements on the left and 1 on the right at line 225, column 45" doesn't help much if you don't know at what rule this happened, especially since the current format doesn't display line and column numbers.

[def-gthill]

Thanks for reporting, I'll make this error message more helpful.

[AdamantConlanger]

thank you for replying so quickly. sorry if it isn't helpful.

[def-gthill]

I've reworked the error reporting system in commit bc900d6aac40101b5b622f181e073d3901642747. In the next release, if you have a "number of elements" mismatch, like this:

bad-rule:
    a => b c

then you'll get an error message like this:

Error in expression 1 ("a => b c") of rule "badrule" Found 1 element ("a") on the left side of the arrow but 2 elements ("b", "c") on the right side

The first line giving the rule name and expression should appear on many other kinds of errors as well.