code: static boolean isValidBase(char base, boolean acceptAmbiguous) {
          boolean isValidBase = PRECISE_BASES.contains(base);
          if (!isValidBase && acceptAmbiguous) {
              isValidBase = N.equals(base) || AMBIGUOUS_BASES.contains(base);
          }
          return isValidBase;
      }
docs: Only accepts as a valid base A, C, G and T
  or IUPAC ambiguous if enabled
  @param base
  @return