1. Method for the construction, within a computer system, of a functional model of a complex electronic system including a plurality of constituents, said functional model including a hierarchised set of modelled constituents, said method comprising: obtaining a multiple instance hierarchical functional model including at least one instance vector corresponding to a plurality of instances of one and the same modelled constituent, each instance vector being able to be located at any level of a hierarchical decomposition tree of said multiple instance hierarchical functional model; expanding said multiple instance hierarchical functional model into an expanded functional model, by expansion of at least one instance vector included in said multiple instance hierarchical functional model, wherein the step of expanding said multiple instance hierarchical functional model includes the following steps, carried out for each instance vector to be expanded corresponding to N instances of a modelled constituent: expanding said instance vector into a number of modelled constituents equal to said number N of instances; and producing the input or inputs and/or the output or outputs of each of the N modelled constituents, according to a set of at least one production rule, wherein said multiple instance hierarchical functional model includes at least one instance vector belonging to the group including: function vectors each corresponding to a plurality of instances of a modelled function; and relationship vectors each corresponding to a plurality of instances of a modelled relationship; and wherein said set of at least one production rule includes the following rules applied to the elementary function vectors each corresponding to a plurality of instances of a modelled elementary function: if, in the multiple instance hierarchical functional model, an elementary function vector output is connected to a relationship vector: if said elementary function vector output is associated with an output selector included in a behavioural model of said modelled elementary function: if the output selector is defined by a parameter indicating assignment in a “1 to 1” relationship, then each instance obtained after expansion of the elementary function vector has an output connected, in said “1 to 1” relationship, to a same index instance among instances obtained after expansion of said relationship vector; and if the output selector is defined by a parameter indicating assignment in a “1 to X” relationship, with X>1, then each instance obtained after expansion of the elementary function vector has X outputs, each connected to a same index instance among the instances obtained after expansion of said relationship vector; and if said elementary function vector output is not associated with an output selector included in a behavioural model of said modelled elementary function, then each instance obtained after expansion of the elementary function vector has an output connected, in a “1 to 1” relationship, to a same index instance among the instances obtained after expansion of said relationship vector.