beam: fix unary-vs-power precedence in expr_eval (-2**2 now -(2**2))

wizard-work/beam/expr_eval.py

@@ -111,7 +111,8 @@ class Parser:
     def parse_unary(self):
         if self.peek()[0] == 'op' and self.peek()[1] in ('+', '-'):
             op = self.advance()[1]
-            v = self.parse_unary()
+            # operand is a full power expression so that -2**2 == -(2**2)
+            v = self.parse_factor()
             return -v if op == '-' else v
         return self.parse_primary()