------
## 最终结论
```python
def evaluate(expression: str) -> int:
    def parse(tokens):
        token = tokens.pop(0)
        if token == '(':
            sub_expr = []
            while tokens[0] != ')':
                sub_expr.append(parse(tokens))
            tokens.pop(0)  # Remove the closing ')'
            return sub_expr
        elif token.isdigit() or (token[0] == '-' and token[1:].isdigit()):
            return int(token)
        else:
            return token

    def eval_expr(expr, scope):
        if isinstance(expr, int):
            return expr
        elif isinstance(expr, str):
            return scope.get(expr, expr)
        
        if expr[0] == 'add':
            _, e1, e2 = expr
            return eval_expr(e1, scope) + eval_expr(e2, scope)
        
        elif expr[0] == 'mult':
            _, e1, e2 = expr
            return eval_expr(e1, scope) * eval_expr(e2, scope)
        
        elif expr[0] == 'let':
            new_scope = scope.copy()
            for i in range(1, len(expr) - 1, 2):
                var = expr[i]
                val = eval_expr(expr[i + 1], new_scope)
                new_scope[var] = val
            return eval_expr(expr[-1], new_scope)

    tokens = expression.replace('(', ' ( ').replace(')', ' ) ').split()
    parsed_expression = parse(tokens)
    return eval_expr(parsed_expression, {})

# Example usage:
# print(evaluate("(add 1 2)"))  # Output: 3
# print(evaluate("(mult 3 (add 2 3))"))  # Output: 15
# print(evaluate("(let x 2 (mult x 5))"))  # Output: 10
# print(evaluate("(let x 2 (mult x (let x 3 y 4 (add x y))))")) # Output: 14
# print(evaluate("(let x 3 x 2 x)")) # Output: 2
# print(evaluate("(let x 1 y 2 x (add x y) (add x y))")) # Output: 5
# print(evaluate("(let x 2 (add (let x 3 (let x 4 x)) x))")) # Output: 6
# print(evaluate("(let a1 3 b2 (add a1 1) b2)")) # Output:4 
```