columns is the bound source's column names, needed for two checks
that are far cheaper here than as a failure later:
a slot may not take the name of a column, because an input string
would then be ambiguous and the column would be shadowed silently
(the alternative, a sigil like "@bb", adds syntax to a format that
has none — [PND-PROCSLOT] prefers the validation);
an input naming neither a slot nor a column is a typo, and saying so
with both lists beats a downstream "column not found".
Expands a slot graph into the nested
Specform.columnsis the bound source's column names, needed for two checks that are far cheaper here than as a failure later:"@bb", adds syntax to a format that has none — [PND-PROCSLOT] prefers the validation);