The AST to ASR has to do something like the following.

• Maintain a hashtable/mapping of labels (strings) -> (unique IDs, bool declared); initialize it as empty when entering a new function
• Every time you encounger a "goto label", lookup the label in the mapping, two cases can happen:
  • if it is already there, use the ID from the mapping and create GoTo(ID)
  • if it is not there, add it to the mapping, and note that it has not been declared yet, and assign a unique ID to it; then use that unique ID in GoTo(ID)
• Every time you encounter "label label", lookup the label in the mapping, two cases can happen:
  • if it is there, change declared to yes, use the ID in GoToTarget(ID)
  • if it is not there, create a new label in the mapping, assign unique ID and declared as yes, then use that ID in GoToTarget(ID)
• at the end of the function, go over this mapping and check that each label has been declared. If there is one or more labels that has not been declared, raise a compiler error. This corresponds to the case of using a "goto label" without having the corresponding "label label" statement.