simp_mode — toggle rational simplification
mod=simp_mode() simp_mode(mod)
rational simplification is called after nearly each operations on
rationals. It is possible to toggle simplification on or off using
simp_mode function.
simp_mod(%t) set rational simplification mode on
simp_mod(%f) set rational simplification mode off
mod=simp_mod() returns in mod the current rational
simplification mode