Challenge (Scrabble)

In the game of Scrabble, a challenge is the act of one player questioning the validity of one or more words formed by another player in the most recent turn. If one or more of the challenged words is not in the previously agreed-upon dictionary, the challenged player loses his turn. If the challenged words are acceptable, the challenger loses his turn.
In tournament play, a player challenges by neutralizing the game clock and announcing, "Challenge." Both players must refer to word judge software, or request an adjudicator if one is unable to do so. Depending on the rules in force, there may be different consequences for a challenge. There are three current variations: single challenge, double challenge and penalty challenge.
Single challenge
In single challenge, if a player places a word and his opponent wishes to challenge, the challenger may do so with no penalty. If the word is valid, it remains on the board. If it is invalid...