diff --git a/stl/ast.py b/stl/ast.py
index 0e80291..8a5f992 100644
--- a/stl/ast.py
+++ b/stl/ast.py
@@ -4,12 +4,9 @@
 
 from collections import namedtuple, deque
 from itertools import repeat
-from typing import Union
 from enum import Enum
 from sympy import Symbol
 
-VarKind = Enum("VarKind", ["x", "u", "w"])
-str_to_varkind = {"x": VarKind.x, "u": VarKind.u, "w": VarKind.w}
 dt_sym = Symbol('dt', positive=True)
 t_sym = Symbol('t', positive=True)
 
@@ -23,12 +20,7 @@ class AtomicPred(namedtuple("AP", ["id"])):
 
 class LinEq(namedtuple("LinEquality", ["terms", "op", "const"])):
     def __repr__(self):
-        n = len(self.terms)
-        rep = "{}"
-        if n > 1:
-            rep += " + {}"*(n - 1)
-        rep += " {op} {c}"
-        return rep.format(*self.terms, op=self.op, c=self.const)
+        return " + ".join(map(str, self.terms)) + f" {self.op} {self.const}"
 
     def children(self):
         return []
@@ -36,13 +28,12 @@ class LinEq(namedtuple("LinEquality", ["terms", "op", "const"])):
 
 class Var(namedtuple("Var", ["coeff", "id", "time"])):
     def __repr__(self):
-        time_str = "[{}]".format(self.time)
-        return "{c}*{i}{t}".format(c=self.coeff, i=self.id, t=time_str)
+        return f"{self.coeff}*{self.id}[{self.time}]"
 
 
 class Interval(namedtuple('I', ['lower', 'upper'])):
     def __repr__(self):
-        return "[{},{}]".format(self.lower, self.upper)
+        return f"[{self.lower},{self.upper}]"
 
     def children(self):
         return [self.lower, self.upper]
@@ -51,14 +42,7 @@ class Interval(namedtuple('I', ['lower', 'upper'])):
 class NaryOpSTL(namedtuple('NaryOp', ['args'])):
     OP = "?"
     def __repr__(self):
-        n = len(self.args)
-        if n == 1:
-            return "{}".format(self.args[0])
-        elif self.args:
-            rep = " {op} ".join(["({})"]*(len(self.args)))
-            return rep.format(*self.args, op=self.OP)
-        else:
-            return ""
+        return f" {self.OP} ".join(f"({x})" for x in self.args)
 
     def children(self):
         return self.args
@@ -72,23 +56,23 @@ class And(NaryOpSTL):
 
 
 class ModalOp(namedtuple('ModalOp', ['interval', 'arg'])):
+    def __repr__(self):
+        return f"{self.OP}{self.interval}({self.arg})"
+    
     def children(self):
         return [self.arg]
 
 
 class F(ModalOp):
-    def __repr__(self):
-        return "◇{}({})".format(self.interval, self.arg)
-
+    OP = "◇"
 
 class G(ModalOp):
-    def __repr__(self):
-        return "□{}({})".format(self.interval, self.arg)
+    OP = "□"
 
 
 class Neg(namedtuple('Neg', ['arg'])):
     def __repr__(self):
-        return "¬({})".format(self.arg)
+        return f"¬({self.arg})"
 
     def children(self):
         return [self.arg]