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)

src/format.rs

@@ -44,7 +44,6 @@ impl Precedence for crate::Term
 			| Self::Function(_)
 			| Self::SpecialInteger(_)
 			| Self::Integer(_)
-			| Self::Symbolic(_)
 			| Self::String(_)
 			| Self::Variable(_)
 				=> 0,
@@ -127,7 +126,6 @@ impl<'term> std::fmt::Debug for TermDisplay<'term>
 			crate::Term::SpecialInteger(crate::SpecialInteger::Infimum) => write!(format, "#inf"),
 			crate::Term::SpecialInteger(crate::SpecialInteger::Supremum) => write!(format, "#sup"),
 			crate::Term::Integer(value) => write!(format, "{}", value),
-			crate::Term::Symbolic(value) => write!(format, "{}", value),
 			crate::Term::String(value) => write!(format, "\"{}\"", value),
 			crate::Term::Variable(variable) => write!(format, "{:?}", variable.declaration),
 			crate::Term::Function(function) =>