Buffer .. var
Buffer concatenation allow to append two buffers to each other, or to concatenate a Buffer with a string (using default UTF8 encoding). The result of the concatenation is a third Buffer.
Example
local buff1 = sys.Buffer { 97, 98, 99 } -- "abc" in utf8
local buff2 = sys.Buffer { 100, 101, 102 } -- "def" in utf8
local concat1 = buff1..buff2
-- outputs 6
print(#concat1)