If none of the other rule types will do, you can define your own
scoring function. “UDF“ stands for user-defined function: such a
function shall take a single parameter of type elfeed-entry and
return either nil (to indicate no interest in this entry) or an
integer (representing this rule’s contribution to this entry’s score).
In the score file (see The Score File), UDF rules are represented by five properties:
:function The user-defined function; typically this will be a
lambda form or a symbol
:display-name elfeed-score will attempt to deduce a name for
the function; if this value is specified, the given string will be
used for logging, pretty-print & so forth
:tags Tag scoping rules
:feeds Feed scoping rules
:comment Comment