Commands into datablocks

[Tait <gnu...@t4...>]

Datablocks seem like a great way to avoid running slow commands 
multiple times, as in:
  plot $data using ..., '' using ..., '' using ...

instead of
  plot '<cmd' using ..., '' using ..., '' using ...

But $data << '<cmd' obviously doesn't work, and the workaround 
using load '<cmd' is hard to discover and requires modifying 
cmd. Would it be straighttforward to add a way to populate a 
datablock from a command the way '<' works for plot and load?

(Inspired by 
https://stackoverflow.com/questions/54600172/gnuplot-load-datafile-11-into-datablock 
and several similar questions. Using system() doesn't work the 
way you'd think either, as 
https://stackoverflow.com/a/60366832/11777995 mentions.)