def add(self, ir="", ia="", ib="", ic="", name="", facta="", factb="",
        factc="", **kwargs):
    """Adds (sums) variables.

    APDL Command: ADD

    Parameters
    ----------
    ir
        Arbitrary reference number assigned to the resulting variable (2 to
        NV [NUMVAR]).  If this number is the same as for a previously
        defined variable, the previously defined variable will be
        overwritten with this result.

    ia, ib, ic
        Reference numbers of the three variables to be operated on.  If
        only two variables, leave IC blank.  If only one, leave IB and IC
        blank.

    name
        Thirty-two character name for identifying the variable on the
        printout and displays.  Embedded blanks are compressed upon output.

    facta, factb, factc
        Scaling factors (positive or negative) applied to the corresponding
        variables (default to 1.0).

    Notes
    -----
    Adds variables (up to three at once) according to the operation:

    IR = (FACTA x IA) + (FACTB x IB) + (FACTC x IC)
    """
    command = f"ADD,{ir},{ia},{ib},{ic},{name},,,{facta},{factb},{factc}"
    return self.run(command, **kwargs)