Concatenation of seq and array?

[PMunch]

Of course writing `s = s & 'x'` is a silly mistake, but if you look at his 
original code snippet `lcuts = v.vertex.neighbors - (lcuts & @[u.vertex, 
w.vertex])` it's not as obvious. And expanding that to more efficient code 
    
    
    lcuts.add u.vertex
    lcuts.add w.vertex
    lcuts = v.vertex.neighbors - lcuts
    
    
    Run

suddenly becomes a question of performance vs. readability.