#!/usr/local/bin/tclsh8.6
set in "pbga (66)
xhth (57)
ebii (61)
havc (66)
ktlj (57)
fwft (72) -> ktlj, cntj, xhth
qoyq (66)
padx (45) -> pbga, havc, qoyq
tknk (41) -> ugml, padx, fwft
jptl (61)
ugml (68) -> gyxo, ebii, jptl
gyxo (61)
cntj (57)"

proc Solve {in} {
    set all {}
    set dropped {}
    foreach line [split $in \n] {
        set words [split $line]
        lappend all [lindex $words 0]
        foreach word [lrange $words 3 end] {
            lappend dropped [string trimright $word ,]
        }
    }
    set all [lsort -unique -dictionary $all]
    set dropped [lsort -unique -dictionary $dropped]
    foreach a $all d $dropped { 
        if {$a ne $d} {
            return $a
        }
    }
}

puts "solution is [Solve $in]"