Provide rationale for one-pass parsing

S02-bits.pod

@@ -43,6 +43,14 @@ Recognize that a reduce operator is not really beginning a C<[...]> composer.
 
 =back
 
+One-pass parsing is fundamental to knowing exactly which language
+you are dealing with at any moment, which in turn is fundamental
+to allowing unambiguous language mutation in any desired direction.
+(Generic languages are allowed, but only if intended; accidentally
+generic languages lead to loss of linguistic identity and integrity.
+This is the hard lesson of Perl 5's source filters and other
+multi-pass parsing mistakes.)
+
 =head1 Lexical Conventions
 
 =head2 Unicode Semantics