forgot to add sugar module + fix breakage due to sugar module

mtl/sugar.py

@@ -0,0 +1,29 @@
+from mtl import ast
+
+
+def alw(phi, *, lo=0, hi=float('inf')):
+    return ast.G(ast.Interval(lo, hi), phi)
+
+
+def env(phi, *, lo=0, hi=float('inf')):
+    return ~alw(~phi, lo=lo, hi=hi)
+
+
+def implies(x, y):
+    return ~x | y
+
+
+def xor(x, y):
+    return (x | y) & ~(x & y)
+
+
+def iff(x, y):
+    return (x & y) | (~x & ~y)
+
+
+def until(phi, psi):
+    return ast.WeakUntil(phi, psi) & env(psi)
+
+
+def timed_until(phi, psi, lo, hi):
+    return env(psi, lo=lo, hi=hi) & alw(until(phi, psi), lo=0, hi=lo)