GNU TeXmacs is a free software for editing scientific documents with mathematical formulas, which can also be used as an interface for many computer algebra systems. We present the design of a new experimental mathematical editing mode which preserves the syntactic correctness of formulas during the editing process (i.e. all formulas can be parsed using a suitable, sufficiently rich grammar). The main constraint is to remain as closely as possible to the existing presentation-oriented formula editor, which has the advantage of being very user friendly.