List of token parsing info.
This list is normally built by `smie-prec2->grammar'.
Each element is of the form (TOKEN LEFT-LEVEL RIGHT-LEVEL).
Parsing is done using an operator precedence parser.
LEFT-LEVEL and RIGHT-LEVEL can be either numbers or a list, where a list
means that this operator does not bind on the corresponding side,
e.g. a LEFT-LEVEL of nil means this is a token that behaves somewhat like
an open-paren, whereas a RIGHT-LEVEL of nil would correspond to something
like a close-paren.