private void mentEndings() {
    int old_k = k;
    
    if (endsIn('m', 'e', 'n', 't')) {
      word.setLength(j + 1);
      k = j;
      if (lookup()) return;
      word.append("ment");
      k = old_k;
      // nolookup
    }
    return;
  }
*
YCS: this was the one place where lookup was not followed by return.
So restructure it. if ((j>0)&&(lookup(word.toString())) &&
!((word.charAt(j) == 's') && (word.charAt(j-1) == 's'))) return;
***
linguistics/src/main/java/com/yahoo/language/simple/kstem/KStemmer.java