constructor(
   public proofTree : IProofTree,
   parent : Maybe<IProofTreeNode>
 ) {
   this.body = undefined
   this.depth = parent.fmap((p : IProofTreeNode) => p.depth + 1).valueOr(0)
   const foo = parent.fmap((p : IProofTreeNode) => p.currentScaledX).valueOr(0)
   this.currentScaledX = parent.fmap(p => p.currentScaledX).valueOr(0)
   this.currentScaledY = parent.fmap(p => p.currentScaledY).valueOr(0)
   this.id = `${nodeId++}` // used to be taken care of by d3
 }

 errorMsg$.subscribe(e => {
   // debugger
   let failedEdit = Maybe.nothing<ISentence<IStage>>()
   switch (e.editOrState) {
     case EditOrState.Edit :
       failedEdit = just(doc.getSentencesToProcess()[0])
       break
     case EditOrState.State :
       failedEdit = doc.getSentenceByStateId(e.editOrStateId)
       break
     default : debugger
   }
   failedEdit.fmap(failedEdit => {
     e.feedbackContent.location.fmap(location => {
       const errorStartIndex = location.bp
       const errorStopIndex = location.ep
       // to compute the document location, we must map the location (nb of characters)
       // to the on-screen position (by virtually moving the cursor right)
       const errorStart = doc.movePositionRight(failedEdit.startPosition, errorStartIndex)
       const errorStop = doc.movePositionRight(failedEdit.startPosition, errorStopIndex)
       const range = new AceAjax.Range(errorStart.row, errorStart.column, errorStop.row, errorStop.column)
       doc.markError(range, clear$)
     })
   })
 })

//////////
function getFirstMemberName(state: State): string | undefined {
	return Maybe.maybe<Data>(state.data!)
	.bind(data => Maybe.maybe(data.members[0]))
	.bind(member => Maybe.just(member.name))
	.caseOf({
		just: name => name,
		nothing: () => undefined
	})

	/*
	return Maybe.maybe<Data>(state.data!)
	.bind(data => Maybe.maybe(data.members[0]))
	.caseOf({
		just: member => member.name,
		nothing: () => undefined
	})
	*/

}

	.bind(member => Maybe.just(member.name))