public boolean containsKey(char[] text, int off, int len) {
    return keys[getSlot(text, off, len)] != null;
  }
true if the <code>len</code> chars of <code>text</code> starting at <code>off</code>
are in the {@link #keySet()}
linguistics/src/main/java/com/yahoo/language/simple/kstem/CharArrayMap.java