{[],
 update(se_lt_x, [c-(C + 2 * F + 20)]),
 @ :: [a-A, c-C] =*= se_lt_y :: [d-A, f-F],
 C < 100}
This joins the table being updated (table se_lt_x) on 
table se_lt_y where se_lt_x.a = se_lt_y.a and 
where se_lt_x.c < 200 then updates each identified row se_lt_x.c 
with the specified expression.