def build_operation_stack(string):
    stack = []
    start = 0
    for i, s in enumerate(string):
        if s == '(':
            substring = string[start:i]
            start = i + 1
            stack.append(substring)
        elif s == ',':
            if i != start:
                substring = string[start:i]
                stack.append(substring)
            start = i + 1
        elif s == ')':
            if i != start:
                substring = string[start:i]
                stack.append(substring)
            start = i + 1
    return stack