mp=: +/ . *  NB. matrix product
   h =: +@|:    NB. conjugate transpose

   QR=: 3 : 0
    n=.{:$A=.y
    if. 1>:n do.
     A ((% {.@,) ; ]) %:(h A) mp A
    else.
     m =.>.n%2
     A0=.m{."1 A
     A1=.m}."1 A
     'Q0 R0'=.QR A0
     'Q1 R1'=.QR A1 - Q0 mp T=.(h Q0) mp A1
     (Q0,.Q1);(R0,.T),(-n){."1 R1
    end.
   )