Answer:
Here's a high-level description of an NFA that accepts strings over the digits 0-9 but does not contain the sequence "777" anywhere in the string:
Start with a single initial state.
For each digit from 0 to 9, create a transition from the initial state to a new state labeled with that digit.
From each state labeled with a digit, create a transition to itself labeled with that same digit.
Create a transition labeled with any digit from each state labeled with a digit to a new state.
From the new state, create a transition labeled with any digit to itself.
Create a transition labeled with any digit from the new state to another new state.
From the second new state, create transitions labeled with 0, 1, 2, 3, 4, 5, 6, and 8 back to the initial state.
Create a transition labeled with 9 from the second new state to a final accepting state.
From the final accepting state, create transitions labeled with any digit back to itself.
This NFA effectively allows any sequence of digits except the sequence "777". It loops back to the initial state whenever it encounters a digit that is not part of "777". If it reaches the end of the string after encountering any other digit, it transitions to the final accepting state.
Please note that while this description provides a high-level idea of the NFA structure, it may be helpful to create a visual representation or a more detailed description with state labels, transition arrows, and specific state transitions if you need to implement this NFA in code or further analyze its behavior.