// D->firstSigDigit equals str.end(), every digit must be a zero and there can
// be at most one dot. On the other hand, if we have a zero with a non-zero
// exponent, then we know that D.firstSigDigit will be non-numeric.
// D->firstSigDigit equals str.end(), every digit must be a zero and there can
// be at most one dot. On the other hand, if we have a zero with a non-zero
// exponent, then we know that D.firstSigDigit will be non-numeric.