docs: public BytecodeVariable getVariable(String variableName, boolean mustExist) {
          if (variableName.equals("this")) return BytecodeVariable.THIS_VARIABLE;
          if (variableName.equals("super")) return BytecodeVariable.SUPER_VARIABLE;
          BytecodeVariable v = (BytecodeVariable) stackVariables.get(variableName);
          if (v == null && mustExist)
              throw new GroovyBugError("tried to get a variable with the name " + variableName + " as stack variable, but a variable with this name was not created");
          return v;
      }