code: protected boolean substitute(final StrBuilder buf, final int offset, final int length) {
          return substitute(buf, offset, length, null) > 0;
      }
docs: Internal method that substitutes the variables.
  <p>
  Most users of this class do not need to call this method. This method will
  be called automatically by another (public) method.
  <p>
  Writers of subclasses can override this method if they need access to
  the substitution process at the start or end.
  
  @param buf  the string builder to substitute into, not null
  @param offset  the start offset within the builder, must be valid
  @param length  the length within the builder to be processed, must be valid
  @return true if altered