In order to allow users to slowly convert to a new syntax tree,
uncompatible with the previous one, Camlp5 can be compiled in
two modes:

  - "transitional": syntax tree is compatible but the new syntax tree
    quotations kit in user syntax q_ast.cmo is not available.

  - "strict": the new syntax tree quotations kit in user syntax q_ast.cmo
    is available but the syntax tree is not compatible with the previous
    versions.

You must choose among these modes at configuration time.

See Camlp5 documentation.