docs: public boolean isPrefixPatternString() {
          String patternString = getValue();
          int length = patternString.length();
          if (length == 0) {
              return false;
          }
          // '_' is not allowed.
          int disallowedWildcardPos = patternString.indexOf('_');
          if (disallowedWildcardPos != -1) {
              return false;
          }
          int firstWildcardPos = patternString.  indexOf('%');
          // Indexable filters have only a trailing '%'.
          // NOTE: not bothering to check for silly synonym patterns with multiple trailing '%'s.
          if (firstWildcardPos != length-1) {
              return false;
          }
          return true;
      }