bug xCas limit

[Albert Chan]

For XCas, replacing variable x to others seems to help.

XCas> f(a) := limit(cos(t+pi/2-a)/(t-a), t=a)
XCas> f(pi/2)   → -1
XCas> f(pi/3)   → -1
XCas> f(0)       → -1

Cas> f(a) := limit(cos(t+pi/2-a)/(t-a), t=a)
Cas> f(pi/2)      → 0
Cas> f(pi/3)      → 0
Cas> f(0)          → -SIN(a)/a       ???