Variant sgr_val x : R -> bool -> bool -> bool -> bool -> bool -> bool
-> bool -> bool -> bool -> bool -> bool -> bool -> R -> Set :=
| SgrNull of x = 0 : sgr_val x 0 true true true true false false
true false false true false false 0
| SgrPos of x > 0 : sgr_val x x false false true false false true
false false true false false true 1
| SgrNeg of x < 0 : sgr_val x (- x) false true false false true false
false true false false true false (-1).

Lemma sgrN x : sg (- x) = - sg x.