I forgot I can remove the outer parentheses, and also use infix with C-like operators, since I have those modes turned on in the REPL. (Infix is an unreleased feature, coming in TXR 300).

  3> for-tree n 1 (if (n < 8) (n << 1)) (if (n < 8) (n << 1 | 1))
       (format t "~b\n" n)
   1000
   100
   1001
   10
   [...]