Modal Reference

Program model
- Program: ordered rules; first match applies.
- Data: atoms and lists (S-expr). Numbers can be encoded as unary parentheses or decimal/hex for devices.
- Registers: ?name binds a subtree in left-hand side and substitutes on the right.

Syntax
- Rule define: (<>) (left) (right)
- Rule undefine: (><) (left) (right)
- Device calls: (?: ...), (?_), (?~), (?^ x), (?. x), (?* x)
- Input lines (loader convenience): lines starting with ".." are expressions appended to the input tree.

Evaluation
1. Scan tree left-to-right (preorder over the AST).
2. At each node:
   a) Dynamic forms: devices and (<>)/ (><) evaluated first.
   b) Rules: try rules in order; on first match, rewrite.
3. After rewrite, restart scanning from root. Halt after a full pass with no rewrites or cycle cap exhaustion.

Matching
- Atom vs atom: equal string.
- List vs list: same arity; element-wise match.
- Register (?x): first occurrence binds; subsequent occurrences must equal the bound value.

Devices
- (?: args...)
  - ALU: (?: op n0 n1 ...), op in + - * / % & ^ | = ! > < ; numbers decimal or #hex.
  - Print: otherwise concatenates args (atoms/lists flattened to string) to stdout; returns ().
- (?_ path)
  - If access allowed: reads file; returns parsed AST if possible else atom of contents; otherwise returns NAF.
- (?~)
  - If access allowed: read stdin; else EOF.
- (?^ x) : join tokens of x (atoms from leaves) → atom.
- (?. x) : unwrap list x → elements of x.
- (?* x) : explode x. If atom, returns list of its characters as atoms. If list, returns x unchanged.

CLI
- -q : quiet (suppress device print output traces except final tree)
- -a : allow access (enables (?_), (?~))
- -n : no cap (large cycle cap)

Notes
- Rule order is significant.
- Dynamic rules enable self-modifying programs.
- Termination is not guaranteed.

Examples
  Rules:
    (<>) (copy ?a) (?a ?a)
    (<>) (swap ?x ?y) (?y ?x)

  Input:
    (copy cat) (swap bat rat)

  Result:
    (cat cat) (rat bat)