integral bug in xCas or HP PRIME

[Albert Chan]

Paradoxically, simple substitution x=s*t eliminated the bug

XCas> f := exp(-s*t)*t*ln(t)
XCas> g := subst(f, t=x/s) / s    // dt = dx/s ⇒ f dt = g dx
XCas> G := int(g, x)                  // G = (-exp(-x)+ln(x/s)*exp(-x)*(-x-1)+Ei(-x))/s^2
XCas> F := subst(G, x=t*s)       // F = (-exp(-s*t)+ln(t)*exp(-s*t)*(-s*t-1)+Ei(-s*t))/s^2
XCas> simplify(diff(F,t) - f)        // 0 ⇒ F = ∫ f dt