Incorrect rendering of variables

[ulrikrasmussen]

The following proof script incorrectly results in the last line in the box being rendered as P(x) when it should be P(y0).

%abbrev
ex2-3-6-a : {P}
proof (all ([x] P x) , |- all ([y] P y)) = 
[P]
all ([x] P x)                           premise; [@prem]
(   var[y0]
    P y0                                by all_e y0 @prem 
)                                       ; [box]
all ([y] P y)                           by all_i box.