Add _symbolunion method

diff --git a/polyp.py b/polyp.py
index 9058c08..ca32493 100644 (file)
--- a/polyp.py
+++ b/polyp.py
@@ -425,8 +425,14 @@ class Polyhedron:
         # return false if the polyhedron is empty, true otherwise
         raise not self.isempty()
 
+    def _symbolunion(self, *others):
+        symbols = set(self.symbols())
+        for other in others:
+            symbols.update(other.symbols())
+        return sorted(symbols)
+
     def __eq__(self, other):
-        symbols = set(self.symbols()) | set(other.symbols())
+        symbols = self._symbolunion(other)
         string = '{} = {}'.format(self._toiscc(symbols), other._toiscc(symbols))
         string = _iscc.eval(string)
         return string == 'True'
@@ -442,7 +448,7 @@ class Polyhedron:
         return (self & other).isempty()
 
     def issubset(self, other):
-        symbols = set(self.symbols()) | set(other.symbols())
+        symbols = self._symbolunion(other)
         string = '{} <= {}'.format(self._toiscc(symbols), other._toiscc(symbols))
         string = _iscc.eval(string)
         return string == 'True'
@@ -451,14 +457,14 @@ class Polyhedron:
         return self.issubset(other)
 
     def __lt__(self, other):
-        symbols = set(self.symbols()) | set(other.symbols())
+        symbols = self._symbolunion(other)
         string = '{} < {}'.format(self._toiscc(symbols), other._toiscc(symbols))
         string = _iscc.eval(string)
         return string == 'True'
 
     def issuperset(self, other):
         # test whether every element in other is in the polyhedron
-        symbols = set(self.symbols()) | set(other.symbols())
+        symbols = self._symbolunion(other)
         string = '{} >= {}'.format(self._toiscc(symbols), other._toiscc(symbols))
         string = _iscc.eval(string)
         return string == 'True'
@@ -467,7 +473,7 @@ class Polyhedron:
         return self.issuperset(other)
 
     def __gt__(self, other):
-        symbols = set(self.symbols() + other.symbols())
+        symbols = self._symbolunion(other)
         string = '{} > {}'.format(self._toiscc(symbols), other._toiscc(symbols))
         string = _iscc.eval(string)
         return string == 'True'
@@ -475,10 +481,7 @@ class Polyhedron:
     def union(self, *others):
         # return a new polyhedron with elements from the polyhedron and all
         # others (convex union)
-        symbols = set(self.symbols())
-        for other in others:
-            symbols.update(other.symbols())
-        symbols = sorted(symbols)
+        symbols = self._symbolunion(*others)
         strings = [self._toiscc(symbols)]
         for other in others:
             strings.append(other._toiscc(symbols))
@@ -490,9 +493,7 @@ class Polyhedron:
         return self.union(other)
 
     def intersection(self, *others):
-        symbols = set(self.symbols())
-        for other in others:
-            symbols.update(other.symbols())
+        symbols = self._symbolunion(*others)
         symbols = sorted(symbols)
         strings = [self._toiscc(symbols)]
         for other in others:
@@ -507,9 +508,7 @@ class Polyhedron:
     def difference(self, *others):
         # return a new polyhedron with elements in the polyhedron that are not
         # in the others
-        symbols = set(self.symbols())
-        for other in others:
-            symbols.update(other.symbols())
+        symbols = self._symbolunion(*others)
         symbols = sorted(symbols)
         strings = [self._toiscc(symbols)]
         for other in others: