Consider the program given below, in a block-structured pseudo-language with lexical scoping and nesting of procedures permitted.

Program main;
  Var . . .

  Procedure A1;
      Var . . .
      Call A2;
  End A1

  Procedure A2;
      Var . . .

      Procedure A21;
        Var . . .
        Call A1;
      End A21

      Call A21;
  End A2

  Call A1;
End main.

Consider the calling chain: Main $$ \to $$ A1 $$ \to $$ A2 $$ \to $$ A21 $$ \to $$ A1
The correct set of activation records along with their access links is given by

Consider the set of strings on $$\left\{ {0,1} \right\}$$ in which, every substring of $$3$$ symbols has at most two zeros. For example, $$001110$$ and $$011001$$ are in the language, but $$100010$$ is not. All strings of length less than $$3$$ are also in the language. A partially completed $$DFA$$ that accepts this language is shown below.

The missing arcs in the $$DFA$$ are

What is the complement of the language accepted by the $$NFA$$ shown below?
Assume $$\sum { = \left\{ a \right\}\,\,} $$ and $$\varepsilon $$ is the empty string.

Which of the following problems are decidable?
$$1.$$ Does a given program ever produce an output?
$$2.$$ If L is a context-free language, then, is $$\overline L $$ also context-free?
$$3.$$ If L is a regular language, then, is $$\overline L $$ also regular?
$$4.$$ If L is a recursive language, then, is $$\overline L $$ also recursive?