theorem Th30: :: JGRAPH_6:30

for a, b, c, d being Real st a <= b & c <= d holds
( LSeg (|[a,c]|,|[a,d]|) = { p1 where p1 is Point of (TOP-REAL 2) : ( p1 `1 = a & p1 `2 <= d & p1 `2 >= c ) } & LSeg (|[a,d]|,|[b,d]|) = { p2 where p2 is Point of (TOP-REAL 2) : ( p2 `1 <= b & p2 `1 >= a & p2 `2 = d ) } & LSeg (|[a,c]|,|[b,c]|) = { q1 where q1 is Point of (TOP-REAL 2) : ( q1 `1 <= b & q1 `1 >= a & q1 `2 = c ) } & LSeg (|[b,c]|,|[b,d]|) = { q2 where q2 is Point of (TOP-REAL 2) : ( q2 `1 = b & q2 `2 <= d & q2 `2 >= c ) } )