code: public String columnPut(String key, String value) {
          checkNotNull(value, "The column expression value must not be null.");
          String resolvedKey = resolveColumnKey(key);
          columnMapCache.clear();
          return columnMap.put(
                  resolvedKey,
                  value.toLowerCase(Locale.ENGLISH));
      }
docs: Associates the specified column String with the specified key in this map. If the map
  previously contained a mapping for this key, the old value is replaced by the specified
  value. The value set in this map overrides an association - if any - in the chain of parent
  LayoutMaps.<p>
  
  The {@code value} must not be {@code null}. To remove an association from this map use
  {@link #columnRemove(String)}.
  
  @param key key with which the specified value is to be associated.
  @param value column expression value to be associated with the specified key.
  @return previous String associated with specified key, or {@code null} if there was no
  mapping for key.
  
  @throws NullPointerException if the {@code key} or {@code value} is {@code null}.
  
  @see Map#put(Object, Object)