ENTER↑ is a misnomer, long live ENTER:

[Thomas Klemm]

From the Hewlett Packard Journal (August 1987, p. 9):

Quote:Of all the decisions made by the design team regarding
the user interface, the one that was by far the most difficult
(as well as the most controversial) was the one that made
the HP-18C operation algebraic, rather than RPN.

The HP-18C has an [INPUT] key.
There's no ENTER command in UserRPL nor in SysRPL. But there's DUP.

Cheers
Thomas