Instead of treating it as a mere whitespace separator. As a result the
lexer output looks like this:

    TOK_1, ..., TOK_N, EOI[, TOK_1, ..., TOK_N, EOI[...]]

At this point though, only the first series of tokens is considered, in
a single query mode that was already what we've had so far. It will also
be the parser's job to ignore consecutive EOI tokens for "empty" queries.