Avoid syntax only supported in newer zsh
    
    zsh previous to 5.0.5 or so doesn't understand an array initializer as
    part of a local declaration:
       local z=(foo)
    
    Split this into local -a z; z=(foo).  Technically the -a isn't required
    even for zsh 4.2 but it's clearer.
    
    Fixes #60.