Oops: prod, not sum:
function Base.reshape{N}(a::AbstractArray, dims::NTuple{N,Union(Int,())})
missing_mask = [isa(x,Tuple) for x in dims]
sum(missing_mask) == 1 || throw(DimensionMismatch("new dimensions
$(dims) may only have up to one omitted dimension"))
sz = length(a) / prod(dims[!missing_mask])
sz == int(sz) || throw(DimensionMismatch("array size $(length(a)) must
be divisible by the product of the new dimensions $(dims[!missing_mask])"))
reshape(a,map(x->isa(x,Tuple) ? int(sz) : x, dims))
end
