def if_then(self, classical_reg, if_program, else_program=None):
        """
        If the classical register at index classical reg is 1, run if_program, else run
        else_program.

        Equivalent to the following construction:

        .. code::

            IF [c]:
               instrA...
            ELSE:
               instrB...
            =>
              JUMP-WHEN @THEN [c]
              instrB...
              JUMP @END
              LABEL @THEN
              instrA...
              LABEL @END

        :param int classical_reg: The classical register to check as the condition
        :param Program if_program: A Quil program to execute if classical_reg is 1
        :param Program else_program: A Quil program to execute if classical_reg is 0. This
            argument is optional and defaults to an empty Program.
        :returns: The Quil Program with the branching instructions added.
        :rtype: Program
        """
        else_program = else_program if else_program is not None else Program()

        label_then = LabelPlaceholder("THEN")
        label_end = LabelPlaceholder("END")
        self.inst(JumpWhen(target=label_then, condition=unpack_classical_reg(classical_reg)))
        self.inst(else_program)
        self.inst(Jump(label_end))
        self.inst(JumpTarget(label_then))
        self.inst(if_program)
        self.inst(JumpTarget(label_end))
        return self
If the classical register at index classical reg is 1, run if_program, else run
        else_program.

        Equivalent to the following construction:

        .. code::

            IF [c]:
               instrA...
            ELSE:
               instrB...
            =>
              JUMP-WHEN @THEN [c]
              instrB...
              JUMP @END
              LABEL @THEN
              instrA...
              LABEL @END

        :param int classical_reg: The classical register to check as the condition
        :param Program if_program: A Quil program to execute if classical_reg is 1
        :param Program else_program: A Quil program to execute if classical_reg is 0. This
            argument is optional and defaults to an empty Program.
        :returns: The Quil Program with the branching instructions added.
        :rtype: Program