gh-38876: minor details in piecewise functions
    
just small changes for better speed maybe

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
    
URL: #38876
Reported by: Frédéric Chapoton
Reviewer(s): gmou3

src/sage/functions/piecewise.py

@@ -673,39 +673,40 @@ def piecewise_add(self, parameters, variable, other):
             funcs = []
             contains_lower = False
             contains_upper = False
-            for i in range(len(points)-1):
+            for i in range(len(points) - 1):
+                a, b = points[i], points[i + 1]
                 try:
-                    contains_lower = (self.domain().contains(points[i]) or
-                        other.domain().contains(points[i])) and not contains_upper
-                    contains_upper = (self.domain().contains(points[i+1]) or
-                        other.domain().contains(points[i+1]))
+                    contains_lower = (self.domain().contains(a) or
+                        other.domain().contains(a)) and not contains_upper
+                    contains_upper = (self.domain().contains(b) or
+                        other.domain().contains(b))
                     if contains_lower:
                         if contains_upper:
-                            rs = RealSet.closed(points[i], points[i+1])
+                            rs = RealSet.closed(a, b)
                         else:
-                            rs = RealSet.closed_open(points[i], points[i+1])
+                            rs = RealSet.closed_open(a, b)
                     else:
                         if contains_upper:
-                            rs = RealSet.open_closed(points[i], points[i+1])
+                            rs = RealSet.open_closed(a, b)
                         else:
-                            rs = RealSet.open(points[i], points[i+1])
-                    point = (points[i+1] + points[i])/2
+                            rs = RealSet.open(a, b)
+                    point = (b + a) / 2
                 except ValueError:
-                    if points[i] == minus_infinity and points[i+1] == infinity:
+                    if a == minus_infinity and b == infinity:
                         rs = RealSet.open(minus_infinity, infinity)
                         point = 0
-                    elif points[i] == minus_infinity:
+                    elif a == minus_infinity:
                         if contains_lower:
-                            rs = RealSet.unbounded_below_closed(points[i+1])
+                            rs = RealSet.unbounded_below_closed(b)
                         else:
-                            rs = RealSet.unbounded_below_open(points[i+1])
-                        point = points[i+1]-1
-                    elif points[i+1] == infinity:
+                            rs = RealSet.unbounded_below_open(b)
+                        point = b - 1
+                    elif b == infinity:
                         if contains_upper:
-                            rs = RealSet.unbounded_above_closed(points[i])
+                            rs = RealSet.unbounded_above_closed(a)
                         else:
-                            rs = RealSet.unbounded_above_open(points[i])
-                        point = points[i]+1
+                            rs = RealSet.unbounded_above_open(a)
+                        point = a + 1
                     else:
                         raise
                 try:
@@ -1160,17 +1161,17 @@ def laplace(self, parameters, variable, x='x', s='t'):
                 (s + 1)*e^(-s)/s^2 + 2*e^(-s)/s - 1/s^2
             """
             from sage.symbolic.assumptions import assume, forget
-            from sage.functions.log import exp
 
             x = SR.var(x)
             s = SR.var(s)
             assume(s > 0)
+            exp_sx = (-s * x).exp()
             result = 0
             for domain, f in parameters:
                 for interval in domain:
                     a = interval.lower()
                     b = interval.upper()
-                    result += (SR(f)*exp(-s*x)).integral(x, a, b)
+                    result += (SR(f) * exp_sx).integral(x, a, b)
             forget(s > 0)
             return result