Trac #19918: doctest fix for: substitute exp(-x) with x infinity fails

{{{
sage: exp(-Infinity)
0
}}}
is fine, but
{{{
sage: exp(-x).subs(x=Infinity)
Traceback (most recent call last):
...
RuntimeError: exp_eval(): exp^(unsigned_infinity) encountered
}}}

Original report thread:

    https://groups.google.com/forum/#!topic/sage-devel/tqDSbZ499ME

URL: http://trac.sagemath.org/19918
Reported by: vdelecroix
Ticket author(s): Benjamin Hackl
Reviewer(s): Ralf Stephan

Author: Release Manager

src/sage/functions/log.py

@@ -119,6 +119,13 @@ def __init__(self):
 
             sage: 2*sqrt(e)
             2*sqrt(e)
+
+        Check that :trac:`19918` is fixed::
+
+            sage: exp(-x^2).subs(x=oo)
+            0
+            sage: exp(-x).subs(x=-oo)
+            +Infinity
         """
         GinacFunction.__init__(self, "exp", latex_name=r"\exp",
                                    conversions=dict(maxima='exp'))

src/sage/functions/trig.py

@@ -604,6 +604,13 @@ def __init__(self):
 
             sage: arctan(x).operator()
             arctan
+
+        Check that :trac:`19918` is fixed::
+
+            sage: arctan(-x).subs(x=oo)
+            -1/2*pi
+            sage: arctan(-x).subs(x=-oo)
+            1/2*pi
         """
         GinacFunction.__init__(self, "arctan", latex_name=r'\arctan',
                 conversions=dict(maxima='atan', sympy='atan'))

src/sage/symbolic/expression.pyx

@@ -1532,6 +1532,17 @@ cdef class Expression(CommutativeRingElement):
             ...               print "deriv: %s, hash:%s"%(deriv,h)
             ...           else:
             ...               hashes.add(n)
+
+        Check whether `oo` keeps its hash in `SR` (:trac:`19918`)::
+
+            sage: hash(oo) == hash(SR(oo))
+            True
+            sage: hash(oo) == hash((-x).subs(x=-oo))
+            True
+            sage: hash(-oo) == hash(SR(-oo))
+            True
+            sage: hash(-oo) == hash((-x).subs(x=oo))
+            True
         """
         return self._gobj.gethash()