Vladimir Propp read a hundred Russian folktales and found they are all the same tale: a fixed alphabet of ~31 narrative functions — villainy, departure, the test, the return — that a story picks from but always in the same ORDER. Slide to assemble a tale, or tap to scramble it and watch the grammar break.
Propp's Morphology of the Folktale (1928): dramatis personae vary, but the FUNCTIONS — the significant acts, numbered — form a fixed inventory that any tale draws from in a constant sequence. A real tale is a strictly increasing subsequence of the function numbers (absence, interdiction, violation, villainy, mediation, departure, struggle, victory, return, recognition, wedding…). The slider builds an in-order tale, then progressively swaps a pair — and the moment two functions fall out of order, it is no longer a well-formed Proppian tale. A fail-loud self-check throws unless the canonical tale is detected in order and a swap is detected as a break.
A representative subset of the 31 functions and the strict-order rule (Propp allowed some moves, inversions and trebling); this is the skeleton, not the whole grammar. The monotone-order test is exact.