The subject says it all: it looks like one can override promote_op to
support the following behaviour:
*julia> **import Base.+*
*julia> **immutable Foo end*
WARNING: Method definition (::Type{Main.Foo})() in module Main at REPL[5]:1
overwritten at REPL[10]:1.
*julia> **+(a::Foo,b::Foo) = 1.0*
*+ (generic function with 164 methods)*
*julia> **Base.promote_op(::typeof(+),::Type{Foo},::Type{Foo}) = Float64*
*julia> **Array(Foo,0) + Array(Foo,0)*
*0-element Array{Float64,1}*
Is this documented somewhere? What if we want to override /, -, etc., is
the solution to write a promote_op for each case?
