the x_coord of breakpoints is unreliable about k-level HOT 1 CLOSED

The yellow and blue lines are the envelopes maintained by two kinetic heap in the klevel alg. At t2 line3 should be inserted into the upper kinetic heap. At this moment the upper kinetic heap only knows what happens in time [-inf,t1]. We must insert line3 first and then delete line2. The problem is when we delete line2 the latest intersection in the upper kinetic heap becomes t1. This is not a valid intersection since at time t1 line1 does not belong to the upper envelope. However if we just discard t1 the kinetic heap refused to do _advance() since the inner trivial kpq will be broken. K-level outputs the lines correctly so i think i won't fix this.

from k-level.