docs: def oper_set_expr(self, ctx, oper, val):
          """
          Returns the MUF needed to do an oper-assign on the lvalue. (+=, etc.)
          """
          if self.readonly:
              raise MuvError(
                  "Cannot assign value to constant '%s'." % self.varname,
                  position=self.position
              )
          varname = ctx.lookup_variable(self.varname)
          if varname is None:
              raise MuvError(
                  "Undeclared identifier '%s'." % self.varname,
                  position=self.position
              )
          if len(self.indexing) == 0:
              if ctx.assign_level > 1:
                  fmt = "{var} @ {val} {oper} dup {var} !"
              else:
                  fmt = "{var} @ {val} {oper} {var} !"
              return fmt.format(
                  var=varname,
                  oper=oper,
                  val=val.generate_code(ctx),
              )
          if len(self.indexing) == 1:
              if ctx.target in ['fb7']:
                  if ctx.assign_level > 1:
                      fmt = (
                          "{var} @ {idx} "
                          "over over [] {val} {oper} "
                          "dup -4 rotate "
                          "-rot ->[] pop"
                      )
                  else:
                      fmt = (
                          "{var} @ {idx} "
                          "over over [] {val} {oper} "
                          "-rot ->[] pop"
                      )
              else:
                  if ctx.assign_level > 1:
                      fmt = (
                          "{var} @ {idx} "
                          "over over [] {val} {oper} "
                          "dup -4 rotate "
                          "rot rot ->[] {var} !"
                      )
                  else:
                      fmt = (
                          "{var} @ {idx} "
                          "over over [] {val} {oper} "
                          "rot rot ->[] {var} !"
                      )
              return fmt.format(
                  var=varname,
                  oper=oper,
                  val=val.generate_code(ctx),
                  idx=self.indexing[0].generate_code(ctx),
              )
          if ctx.target in ['fb7']:
              if ctx.assign_level > 1:
                  fmt = (
                      "{var} @ {{ {sidx} }}list array_nested_get "
                      "{lidx} over over [] {val} {oper} "
                      "dup -4 rotate "
                      "-rot ->[] pop"
                  )
              else:
                  fmt = (
                      "{var} @ {{ {sidx} }}list array_nested_get "
                      "{lidx} over over [] {val} {oper} "
                      "-rot ->[] pop"
                  )
          else:
              if ctx.assign_level > 1:
                  fmt = (
                      "{var} @ {{ {idx} }}list "
                      "over over array_nested_get {val} {oper} "
                      "dup -4 rotate "
                      "rot rot array_nested_set {var} !"
                  )
              else:
                  fmt = (
                      "{var} @ {{ {idx} }}list "
                      "over over array_nested_get {val} {oper} "
                      "rot rot array_nested_set {var} !"
                  )
          return fmt.format(
              var=varname,
              oper=oper,
              val=val.generate_code(ctx),
              idx=" ".join(
                  x.generate_code(ctx)
                  for x in self.indexing
              ),
              sidx=" ".join(
                  x.generate_code(ctx)
                  for x in self.indexing[:-1]
              ),
              lidx=self.indexing[-1].generate_code(ctx)
          )