Num.pos constructor
Constrains data to positive numbers (zero included).
mal
"malformed", the error message in case of non-numeric input values;
the default value is 'not a number'.
neg
the error message if an input value is negative; the default value
is 'it cannot be negative'.
Implementation
Num.pos({String? mal, String? neg})
: this.min(0, mal: mal, small: neg ?? 'it cannot be negative');