public Assignment(String booleanAssign){
  this();
  addPair(booleanAssign);
}

public Assignment ( String booleanAssign ) { this ( ) ; addPair ( booleanAssign ) ; }
creates an assignment with a single pair , given a boolean assignment such as " variable " or " ! variable " . if booleanassign start with an exclamation mark , the value is set to false , else the value is set to true .
