def set_of_vars(arg_plot):
sovs = set(tuple((var + '<STR_LIT:+>').split('<STR_LIT:+>')[:<NUM_LIT:2>])<EOL>for var in arg_plot.split('<STR_LIT:U+002C>'))<EOL>sovs.discard(('<STR_LIT>', '<STR_LIT>'))<EOL>return sovs<EOL>
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.