---
doc/src/modules/logic.txt | 7 ++++---
sympy/logic/boolalg.py | 12 +++++++++++-
sympy/logic/tests/test_boolalg.py | 6 ++++--
3 files changed, 19 insertions(+), 6 deletions(-)
diff --git a/doc/src/modules/logic.txt b/doc/src/modules/logic.txt
index 2487d57..e466d5a 100644
--- a/doc/src/modules/logic.txt
+++ b/doc/src/modules/logic.txt
@@ -24,12 +24,13 @@ You can build boolean expressions with the standard python
operators & (And),
>>> ~x
Not(x)
-You can also form implications with >>, <<, and class Equivalent:
+You can also form implications with >>, <<, and class Equivalent, which are
+compiled away into their logical equivalents:
>>> x >> y
- Implies(x, y)
+ Or(Not(x), y)
>>> x << y
- Implies(y, x)
+ Or(Not(y), x)
Like most types in SymPy, Boolean expressions inherit from sympy.core.Basic:
diff --git a/sympy/logic/boolalg.py b/sympy/logic/boolalg.py
index cd979bd..0a45e42 100755
--- a/sympy/logic/boolalg.py
+++ b/sympy/logic/boolalg.py
@@ -104,7 +104,17 @@ def eval(cls, *args):
return A
class Implies(BooleanFunction):
- pass
+ """Logical implication.
+ A implies B is equivalent to !A v B
+ """
+ @classmethod
+ def eval(cls, *args):
+ if len(args) < 2:
+ raise ValueError, "Only %d operand(s) used for an Implies (pairs
are required): %s" % (len(args), str(args))
+ elif len(args) > 2:
+ return map(cls, args)
+ else:
+ return Or(Not(args[0]), args[1])
class Equivalent(BooleanFunction):
"""Equivalence relation.
diff --git a/sympy/logic/tests/test_boolalg.py
b/sympy/logic/tests/test_boolalg.py
index 3102f06..e28f063 100755
--- a/sympy/logic/tests/test_boolalg.py
+++ b/sympy/logic/tests/test_boolalg.py
@@ -97,8 +97,10 @@ def test_Nor():
def test_Implies():
A, B, C = symbols('ABC')
- Implies(True, True) == True
- Implies(False, False) == False
+ assert Implies(True, True) == True
+ assert Implies(True, False) == False
+ assert Implies(False, True) == True
+ assert Implies(False, False) == True
assert A >> B == B << A
def test_Equivalent():
--
1.6.3.3
--
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