code: @SuppressWarnings("unchecked")
  	public <T> T defineContextualValue(String key, Object value) {
  		return (T) this.values.put(key,  value);
  	}
docs: Define a contextual value.
  
  @param <T> the type of the value.
  @param key the name of the value.
  @param value the value itself.
  @return the previous value associated to the key, or {@code null} if none.