Remove unnecessary Symbolic type

This type is a special case of Function and doesn’t need to be
represented separately.

src/ast.rs

@@ -356,7 +356,6 @@ pub enum Term
 	Integer(i32),
 	SpecialInteger(SpecialInteger),
 	String(String),
-	Symbolic(String),
 	UnaryOperation(UnaryOperation),
 	Variable(Variable),
 }
@@ -450,11 +449,6 @@ impl Term
 		Self::special_integer(SpecialInteger::Supremum)
 	}
 
-	pub fn symbolic(value: String) -> Self
-	{
-		Self::Symbolic(value)
-	}
-
 	pub fn true_() -> Self
 	{
 		Self::boolean(true)