code: func ValidateNonnegativeField(value int64, fldPath *field.Path) field.ErrorList {
  	allErrs := field.ErrorList{}
  	if value < 0 {
  		allErrs = append(allErrs, field.Invalid(fldPath, value, IsNegativeErrorMsg))
  	}
  	return allErrs
  }
docs: // Validates that given value is not negative.