Continue parsing formulas

src/ast.rs

@@ -29,6 +29,15 @@ pub enum UnaryOperator
 	Negative,
 }
 
+// ImplicationDirection
+
+#[derive(Clone, Copy, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub enum ImplicationDirection
+{
+	LeftToRight,
+	RightToLeft,
+}
+
 // Primitives
 
 #[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
@@ -291,38 +300,22 @@ impl ForAll
 	}
 }
 
-#[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
-pub struct IfAndOnlyIf
-{
-	pub left: Box<Formula>,
-	pub right: Box<Formula>,
-}
-
-impl IfAndOnlyIf
-{
-	pub fn new(left: Box<Formula>, right: Box<Formula>) -> Self
-	{
-		Self
-		{
-			left,
-			right,
-		}
-	}
-}
-
 #[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct Implies
 {
+	pub direction: ImplicationDirection,
 	pub antecedent: Box<Formula>,
 	pub implication: Box<Formula>,
 }
 
 impl Implies
 {
-	pub fn new(antecedent: Box<Formula>, implication: Box<Formula>) -> Self
+	pub fn new(direction: ImplicationDirection, antecedent: Box<Formula>, implication: Box<Formula>)
+		-> Self
 	{
 		Self
 		{
+			direction,
 			antecedent,
 			implication,
 		}
@@ -480,7 +473,7 @@ pub enum Formula
 	Compare(Compare),
 	Exists(Exists),
 	ForAll(ForAll),
-	IfAndOnlyIf(IfAndOnlyIf),
+	IfAndOnlyIf(Formulas),
 	Implies(Implies),
 	Not(Box<Formula>),
 	Or(Formulas),
@@ -542,14 +535,17 @@ impl Formula
 		Self::compare(ComparisonOperator::GreaterOrEqual, left, right)
 	}
 
-	pub fn if_and_only_if(left: Box<Formula>, right: Box<Formula>) -> Self
+	pub fn if_and_only_if(arguments: Formulas) -> Self
 	{
-		Self::IfAndOnlyIf(IfAndOnlyIf::new(left, right))
+		assert!(!arguments.is_empty());
+
+		Self::IfAndOnlyIf(arguments)
 	}
 
-	pub fn implies(antecedent: Box<Formula>, consequent: Box<Formula>) -> Self
+	pub fn implies(direction: ImplicationDirection, antecedent: Box<Formula>,
+		consequent: Box<Formula>) -> Self
 	{
-		Self::Implies(Implies::new(antecedent, consequent))
+		Self::Implies(Implies::new(direction, antecedent, consequent))
 	}
 
 	pub fn less(left: Box<Term>, right: Box<Term>) -> Self