parameters: (x,y,board)
return_statement: return False
docstring: A utility function to check if i,j are valid indexes 
  		for N*N chessboard
docstring_summary: A utility function to check if i,j are valid indexes 
  		for N*N chessboard
docstring_tokens: A utility function to check if i j are valid indexes for N * N chessboard
function: def isSafe(x,y,board): 
  	''' 
  		A utility function to check if i,j are valid indexes 
  		for N*N chessboard 
  	'''
  	if(x >= 0 and y >= 0 and x < n and y < n and board[x][y] == -1): 
  		return True
  	return False
function_tokens: def isSafe ( x , y , board ) : if ( x >= 0 and y >= 0 and x < n and y < n and board [ x ] [ y ] == - 1 ) : return True return False