feat: new engine format

internal/registry/repr.go

@@ -1,11 +1,14 @@
 package registry
 
+// A Repr is a lambda calculus expression. It can have any type of
+// representation, so long as that class is known to the registry it is handled
+// by.
 type Repr interface {
-	// Id returns to name of the objects underlying representation. If is
-	// assumed that if two Repr objects have the same Id(), they share the same
-	// representation.
+	// ID returns the name of the underlying representation. It is assumed that
+	// if two expressions have the same Id(), they have the same representation.
 	ID() string
 
+	// The base expression data.
 	Data() any
 }