The EXISTS expression is a bit of an odd-man-out because it can appear
as any of the following forms:
EXISTS (select-stmt)
NOT EXISTS (select-stmt)
(select-stmt)
Which makes it the only keyword expression that doesn't require its
keyword to actually be used. The consequence is that we might come
across an EXISTS expression while parsing another expression type;
NOT would have triggered a unary operator expression, and an opening
parentheses would have triggered an expression chain.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
https://sqlite.org/lang_expr.html
The entry point to using expressions, parse_expression(), is not used
by SQL::Parser in this commit. But there's so much here that it's easier
to grok as its own commit.
