Performance regression / unexpected nontermination

Like #355, this is an issue with [`presprover.pl`](https://www.metalevel.at/presprover/presprover.pl): I expect for example the following queries to terminate instantly, but currently they loop: <pre> <b>?- valid(exists(x, x &gt; 0)).</b> loops, unexpected. <b>?- valid(forall(x, exists(y, 3*x + y &gt; 2))).</b> loops, unexpected. </pre> This *used to work as expected* at one point!