Help with `Base.getproperty` syntax

General Usage
2 
julia> struct S
           x
       end

julia> function Base.getproperty(x::S, sym::Symbol)
           if sym == :a
               "a"
           else
               # fallback to getfield
               getfield(x, sym)
           end
       end

julia> obj = S(1)
S(1)

julia> obj.a
"a"

julia> obj.x
1

julia> obj.y
ERROR: type S has no field y
Stacktrace:
 [1] getproperty(::S, ::Symbol) at ./REPL[2]:6
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