1. A method of merging two arbitrarily interconnected Mixed Logical Dynamic (MLD) subsystems (S 1 , S 2 ) into one combined MLD system (S), wherein the combined MLD system (S) represents an industrial process, each MLD subsystem (S 1 , S 2 ) represents a sub-process of the industrial process, the MLD subsystems (S 1 , S 2 ) and the combined MLD system (S) each involve vectors of states ( x 1 , x 2 , x ), vectors of inputs ( u 1 , u 2 , u ), vectors of outputs ( y 1 , y 2 , y ), real auxiliary variables ( z 1 , z 2 , z ) and Boolean auxiliary variables ( δ 1 , δ 2 , δ ), and are described by a state equation, an output equation and constraints, the two subsystems (S 1 , S 2 ) are interconnected via dependent pairs (d y 1 , d u 2 ; d y 2 , d u 1 ) formed by a dependent output (d y 1 , d y 2 ) of one subsystem (S 1 , S 2 ) and a dependent input (d u 2 , d u 1 ) of the other subsystem (S 2 , S 1 ), and independent inputs (i u 1 i y 2 ) and outputs (i y 1 , i y 2 ) of the subsystems (S 1 , S 2 ) are not part of the dependent pairs (d y 1 , d u 2 ; d y 2 , d u 1 ), and wherein the state vector ( x ) of the combined system (S) is composed of the corresponding states ( x 1 ; x 2 ) of the subsystems (S 1 , S 2 ), the input and output vectors ( u , y ) of the combined system (S) are composed of the corresponding independent inputs (i u 1 , i y 2 ) and outputs (i y 1 , i y 2 ) of the subsystems (S 1 , S 2 ), the auxiliary variables ( z , δ ) of the combined system (S) comprise the corresponding auxiliary variables ( z 1 , δ 1 ; z 2 , δ 2 ) of the subsystems (S 1 , S 2 ), each dependent pair (d y 1 , d u 2 ; d y 2 , d u 1 ) is replaced by an additional auxiliary real and/or Boolean variable ( z 12 , z 21 , δ 12 , δ 21 ) of the combined system (S), the output equations for the dependent outputs (d y 1 , d y 2 ) of each subsystem (S 1 , S 2 ) are converted into additional constraints of the combined system (S) involving the additional auxiliary real and/or Boolean variables ( z 12 , z 21 , δ 12 , δ 21 ).