code: func If(conditional BoolExpression,
  	trueExpression Expression,
  	falseExpression Expression) Expression {
  	return &ifExpression{
  		conditional:     conditional,
  		trueExpression:  trueExpression,
  		falseExpression: falseExpression,
  	}
  }
docs: // Returns a representation of an if-expression, of the form:
  //   IF (BOOLEAN TEST, VALUE-IF-TRUE, VALUE-IF-FALSE)