public static bool TryMatchVarPlusConst <Variable, Expression>(this IExpressionDecoder <Variable, Expression> decoder,
                                                                       Expression exp, out Variable var, out int k)
        {
            Contract.Requires(decoder != null);

            if (decoder.IsBinaryExpression(exp) && decoder.OperatorFor(exp) == ExpressionOperator.Addition)
            {
                var left  = decoder.LeftExpressionFor(exp);
                var right = decoder.RightExpressionFor(exp);

                if (decoder.IsConstantInt(left, out k) && decoder.IsVariable(right))
                {
                    var = decoder.UnderlyingVariable(right);

                    return(true);
                }

                if (decoder.IsConstantInt(right, out k) && decoder.IsVariable(left))
                {
                    var = decoder.UnderlyingVariable(left);

                    return(true);
                }
            }

            var = default(Variable);
            k   = default(int);
            return(false);
        }