An internal assembly comprising: a primary module (13), comprising a primary liquid inlet (21), an auxiliary liquid inlet (23), a primary liquid outlet (25) and a mixer (17); and a secondary module (15), comprising a secondary liquid inlet (78), a subsidiary liquid inlet (54), a secondary liquid outlet (64) and a regulator (56). According to the invention, the internal assembly comprises a coupler (40, 42, 70, 80): which connects the primary output (25) with the subsidiary input (54); which connects the secondary input (78) with the auxiliary input (23); and which allows the primary module (13) to slide relative to the secondary module (15) parallel to a sliding axis (X13) of the internal assembly.
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