I'm trying to transcribe the FMT function from Formatting APL to Dyalog.
However I'm not sure of the meaning of the underbar S and I don't see a mechanism to enter underbar variable names into Dyalog.
So two questions in one: How does one enter underbar variable names into Dyalog and do they have some implied meaning (eg transpose of S)?
Underscored letters are just standard variables - documentation here- although typically replaced in the font with circled letters.
Whilst I'm not sure how to type them on other platforms, in the terminal you can layer the _ and S together in replace mode - typically toggled to by the Insert key (IN keycode). So type _ then left arrow to move cursor over the '_' and then the letter (S).
⎕FMT can also generate these with {⎕FMT '_',(⎕UCS 8),⍵}'S'