-----Description-----  
This task requires writing a Lean 4 method that inserts a subarray of characters into another array of characters at a specified index. The method takes the original array (oline) and, given the effective length l to consider, inserts another array of characters (nl) of effective length p starting at the index atPos. The resulting array is of length l + p and is constructed as follows:
• All characters before the insertion position (atPos) remain unchanged.  
• The new characters from nl are inserted starting at index atPos.  
• The remaining characters from the original array (starting at atPos) are shifted right by p positions.

-----Input-----  
The input consists of:  
• oline: An array of characters representing the original sequence.  
• l: A natural number indicating how many characters from oline to consider.  
• nl: An array of characters to be inserted into oline.  
• p: A natural number indicating how many characters from nl to consider for insertion.  
• atPos: A natural number indicating the position in oline where the insertion should occur (0-indexed).

-----Output-----  
The output is an array of characters that is the result of inserting nl into oline at the given atPos. Specifically, the output array should:  
• Contain the original characters from index 0 up to (but not including) atPos.  
• Have the next p characters equal to the characters from nl.  
• Contain the remaining characters from oline (starting from atPos) shifted right by p positions.

-----Note-----  
It is assumed that:  
• atPos is within the range [0, l].  
• l does not exceed the size of oline.  
• p does not exceed the size of nl.