code: def get_context(line, pos):
          
          commands = split_commandline(line) + ['']
          i = 0
          start = 0
          end = len(commands[i])
          while pos > end:
              i += 1
              start = end + 1
              end += 1 + len(commands[i])
          return start, end
docs: computes start and end position of substring of line that is the
          command string under given position