docs: def set_expr(self, ctx):
          """
          Returns the MUF needed to do an assign on the lvalue. (=)
          Returned MUF expects a value to be on the stack.
          """
          if self.readonly:
              raise MuvError(
                  "Cannot assign value to constant '%s'." % self.varname,
                  position=self.position
              )
          if self.declare:
              varname = ctx.declare_variable(self.varname)
              if len(self.indexing) == 0:
                  return "var! {var}".format(var=varname)
          else:
              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:
                  return "dup {var} !".format(var=varname)
              else:
                  return "{var} !".format(var=varname)
          if len(self.indexing) == 1:
              if ctx.target in ['fb7']:
                  if ctx.assign_level > 1:
                      fmt = "dup {var} @ {idx} ->[] pop"
                  else:
                      fmt = "{var} @ {idx} ->[] pop"
              else:
                  if ctx.assign_level > 1:
                      fmt = "dup {var} @ {idx} ->[] {var} !"
                  else:
                      fmt = "{var} @ {idx} ->[] {var} !"
              return fmt.format(
                  var=varname,
                  idx=self.indexing[0].generate_code(ctx),
              )
          if ctx.target in ['fb7']:
              if ctx.assign_level > 1:
                  fmt = "dup {var} @ {{ {idx} }}list array_nested_set pop"
              else:
                  fmt = "{var} @ {{ {idx} }}list array_nested_set pop"
          else:
              if ctx.assign_level > 1:
                  fmt = "dup {var} @ {{ {idx} }}list array_nested_set {var} !"
              else:
                  fmt = "{var} @ {{ {idx} }}list array_nested_set {var} !"
          return fmt.format(
              var=varname,
              idx=" ".join(x.generate_code(ctx) for x in self.indexing)
          )