There are two buffers, IBUF (insert buffer) and FBUF (find
buffer). They are used as a typing target for the actions
described below. They both have a peculiar logic: when typing
is expected to fill a buffer, an empty value means "reuse
previous value". For example, typing "I foo I " inserts
"foofoo".

T ( n -- ): select line n for editing.
P xxx: put typed IBUF on selected line.
U xxx: insert typed IBUF on selected line.
F xxx: find typed FBUF in block, starting from current
       position+1. If not found, don't move.
I xxx: insert typed IBUF at cursor. "I" shadows core word. Use
       "i" to access it.
X ( n -- ): Delete X chars after cursor and place in IBUF.
E: Run X with n = length of FBUF.
