f3 = {__index = {}}
f4 = {__index = {}}
f3.__index.cross = function(a, b) return "cross 3" end
f4.__index.cross = function(a, b) return "cross 4" end
 
function float3() local ob = {}; setmetatable(ob, f3); return ob end
function float4() local ob = {}; setmetatable(ob, f4); return ob end
 
function is_compatible(a, b)
	return getmetatable(a) == getmetatable(b)
end
 
function cross(a, b)
	if not is_compatible(a, b) then return "error" end
	return a.cross(b)
end
 
print("wariant 33: " .. cross(float3(), float3()))
print("wariant 44: " .. cross(float4(), float4()))
print("wariant 34: " .. cross(float3(), float4()))
print("wariant 43: " .. cross(float4(), float3()))

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