beam: fix safe_eval chained unary signs (--2, +-3) by recursing factor

wizard-work/beam/safe_eval.py

@@ -164,7 +164,7 @@ class _Parser:
     def factor(self):
         if self.peek()[0] == _OP and self.peek()[1] in ("+", "-"):
             op = self.advance()[1]
-            val = self.power()  # unary lower precedence than ** (python semantics)
+            val = self.factor()  # recurse so chained signs (--2, +--3) parse
             return -val if op == "-" else val
         return self.power()