An exists restriction translates to a WHERE sub-query
and is used to say that "each row returned in the main query must
satisfy some condition expressed by another query".
Example
{[],
se_lt_x :: [a-A, b-B],
exists se_lt_y :: [d-A]}
compiles to:
SELECT x.b, x.a FROM se_lt_x x WHERE EXISTS (SELECT * FROM se_lt_y WHERE x.a = y.d)