fix script parse error while 'A==-1'

Author: wangzheng

examples/t09_scripting.cpp

@@ -10,7 +10,7 @@ static const char* xml_text = R"(
         <Sequence>
             <Script code=" msg:='hello world' " />
             <Script code=" A:=THE_ANSWER; B:=3.14; color:=RED " />
-            <Precondition if="A>B && color != BLUE" else="FAILURE">
+            <Precondition if="A>-B && color != BLUE" else="FAILURE">
                 <Sequence>
                   <SaySomething message="{A}"/>
                   <SaySomething message="{B}"/>

include/behaviortree_cpp/scripting/operators.hpp

@@ -796,9 +796,9 @@ struct Expression : lexy::expression_production
                                dsl::op<Ast::ExprComparison::greater_equal>(LEXY_LIT(">"
                                                                                     "="));
 
-    // The use of dsl::groups ensures that an expression can either contain math or bit
+    // The use of dsl::groups ensures that an expression can either contain math or bit or string
     // operators. Mixing requires parenthesis.
-    using operand = dsl::groups<math_sum, bit_or>;
+    using operand = dsl::groups<math_sum, bit_or, string_concat>;
   };
 
   // Logical operators,  || and &&
@@ -808,7 +808,7 @@ struct Expression : lexy::expression_production
         dsl::op<Ast::ExprBinaryArithmetic::logic_or>(LEXY_LIT("||")) /
         dsl::op<Ast::ExprBinaryArithmetic::logic_and>(LEXY_LIT("&&"));
 
-    using operand = dsl::groups<string_concat, comparison>;
+    using operand = comparison;
   };
 
   // x ? y : z