es-first-from(es;e;l;tg) == hd(filter(r.tag(kind(r)) = tg;es-receives(es;e;l))) 

latex

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clarification:

es-first-from(es;e;l;tg) == hd(filter(r.tag(es-kind(es; r)) = tg;es-receives(es;e;l))) 

latex

Definitions	hd(l), filter(P;l), x.A(x), a = b, tag(k), kind(e), es-receives(es;e;l)
FDL editor aliases	es-first-from