A "friend" asks you to construct the phase diagram for a new binary alloy of A and B. The clues are: (i) A and B are completely immiscible in each other and no solid solutions exist in the diagram, (ii) no stoichiometric compounds exist in the diagram, (iii) pure A and pure B melt at 1500 and 1200°C, respectively, (iv) pure A does not undergo any phase transitions before it melts, but pure B transforms into a high temperature polymorph at 300°C, which is stable until the melting temperature of B, and (v) at the composition 70 at% B and 700°C, a homogeneous liquid solidifies directly to a solid containing A and the high temperature polymorph of B. Construct the phase diagram.