code: final boolean matchesIndexValue(int v) {
          // Delegate to the key, or if there is none, to the postContext.
          // If there is neither then we match any key; return true.
          UnicodeMatcher m = (key != null) ? key : postContext;
          return (m != null) ? m.matchesIndexValue(v) : true;
      }
docs: Internal method.  Returns true if this rule matches the given
  index value.  The index value is an 8-bit integer, 0..255,
  representing the low byte of the first character of the key.
  It matches this rule if it matches the first character of the
  key, or if the first character of the key is a set, and the set
  contains any character with a low byte equal to the index
  value.  If the rule contains only ante context, as in foo)>bar,
  then it will match any key.