validate method
Implementation
String? validate(String unit) {
assert(unit.isNotEmpty,
paramError('validate', 'unit', 'must not be null or empty'));
try {
ExpressionParser(model).parse(unit);
return null;
} catch (e) {
if (e is UcumException) {
return e.message;
} else {
return e.toString();
}
}
}