FirstD now returns 0 if value can't be found

(Rather than raising an error.)

lucas.py

@@ -82,7 +82,8 @@ def Jacobi(a, n):
 
 def FirstD(n):
     """Return first D in the sequence 5, -7, 9, -11, 13, -15... for which the
-    Jacobi symbol (D/n) is -1
+    Jacobi symbol (D/n) is -1. Returns 0 if the number is a perfect square, or
+    otherwise the value can't be found
     """
     if n < 1 or n % 2 == 0:
         raise ValueError("n must be a positive odd number")
@@ -95,7 +96,7 @@ def FirstD(n):
         # This is supposed to be faster
         if D > 30 and not haschecked:
             if hasIntSQRT(n):
-                raise ValueError("n must not be a square")
+                return 0
     # Shouldn't fire
     return 0