Concatenation of seq and array?

[Zoom]

Perhaps the compiler could track situations like 
    
    
    var x
    x = x & y
    
    
    Run

where T of y is T of x[T] or y is a view of a container [T] and replace the 
second line with appropriate `x.add(y)` or `x.extend(y)`, but do we really want 
this complex behaviour which encourages ambiguous expressions in the code? I 
don't think so.