* It returns the corresponding LogicalExpression
*/
static private LogicalExpression relation(TermSystem p,String op,Expression first,Expression second) {
LogicalExpression result=null;
if (isEqual(op)) {
result = new EqualExpression(first,second);
}
else if (isLess(op))
result = new LessExpression(first,second);
else if (isLessEqual(op))
result = new LessEqualExpression(first,second);