docs: def set_of_vars(arg_plot):
      """Build set of needed field variables.
  
      Each var is a tuple, first component is a scalar field, second component is
      either:
  
      - a scalar field, isocontours are added to the plot.
      - a vector field (e.g. 'v' for the (v1,v2,v3) vector), arrows are added to
        the plot.
  
      Args:
          arg_plot (str): string with variable names separated with
              ``,`` (figures), and ``+`` (same plot).
      Returns:
          set of str: set of needed field variables.
      """
      sovs = set(tuple((var + '+').split('+')[:2])
                 for var in arg_plot.split(','))
      sovs.discard(('', ''))
      return sovs