$a$.$f$($a$) is c$<$ preserving on $e$.$P$($e$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$e$, ${\it e'}$:\{$e$:E$\mid$ $P$($e$)\} . ($e$ $<$ ${\it e'}$) $\Rightarrow$ ($f$($e$) $<$ $f$(${\it e'}$))