it was a normal problem , an expression containing xs and ys and zs , coupled with several integers , two of which were negative ; it was a normal mathematics problem save for one symbol , one that , prior to several weeks ago , melody had never seen before and struggled to believe actually existed , a symbol that was intensely sharp and radiant when first apprehended , startling her such , but one that would then quickly fade in and out of focus , as if seen through a liquid curtain of burning tears .