/-  spider
/+  strandio
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
=+  !<([pax=path ~] arg)
?~  bem=(de-beam:format pax)
  (strand-fail:strand %path-not-beam >pax< ~)
=/  =mark  (rear s.u.bem)
;<  =dais:clay  bind:m  (build-mark:strandio -.u.bem mark)
(pure:m !>(dais))