Scilab automatically performs pole-zero simplifications when the
the built-in primitive `simp` finds a common factor in the
numerator and denominator of a rational polynomial `num/den`.
Pole-zero simplification is a difficult problem from a
numerical viewpoint and `simp` function is usually conservative.
When making calculations with polynomials, it is sometimes desirable
to avoid pole-zero simplifications: this is possible by switching
Scilab into a ``no-simplify'' mode: `help simp_mode`

. The
function `trfmod` can also be used for simplifying specific
pole-zero pairs.