1. A computer-implemented method of collaborative design for merging a first modeled object with a second modeled object wherein the first and second modeled objects are modified copies of an initial modeled object, wherein the first modeled object, the second modeled object and the initial modeled object are CAD three-dimensional modeled objects defined respectively by an initial graph (A), a first graph (B) and a second graph (B′), wherein the initial graph (A), the first graph (B) and the second graph (B′) are boundary representation graphs or history graphs, wherein the method comprises: providing the initial modeled object; providing a respective copy of the initial modeled object to each of a first user and a second user, with at least one processor: performing, by the first user, a design modification on the copy of the initial modeled object respective to the first user, thereby providing the first modeled object, performing, by the second user, a design modification on the copy of the initial modeled object respective to the second user, the design modification performed by the second user being different from the design modification performed by the first user, thereby providing the second modeled object, computing a first double push-out graph rewriting rule and a second double push-out graph rewriting rule, the first double push-out graph rewriting rule and the second double push-out graph rewriting rule corresponding respectively to a transformation of the initial graph (A) into the first graph (B) and the second graph (B′), wherein the first double push-out graph rewriting rule specifies a first part (L) from the initial graph (A) which is to be replaced when transforming the initial graph (A) into the first graph (B), a first replacement (R) to replace the first part (L), a first interface (G) between the first part (L) and the first replacement (R), first morphisms from the first interface (G) to the first part (L) and to the first replacement (R) respectively, and wherein the second double push-out graph rewriting rule specifies a second part (L′) from the initial graph (A) which is to be replaced when transforming the initial graph (A) into the second graph (B′), a second replacement (R′) to replace the second part (L′), a second interface (G′) between the second part (L′) and the second replacement (R′), second morphisms from the second interface (G′) to the second part (L′) and to the second replacement (R′) respectively; assembling the first rewriting rule and the second rewriting rule to determine and provide a third double push-out graph rewriting rule, wherein the third double push-out graph rewriting rule specifies an assembled part ({tilde over (L)}) which is an assembly of the first part (L) and the second part (L′), an assembled replacement ({tilde over (R)}) which is an assembly of the first replacement (R) and the second replacement (R′), an assembled interface ({tilde over (G)}) which is an assembly of the first interface (G) and the second interface (G′), assembled morphisms from the assembled interface ({tilde over (G)}) to the assembled part ({tilde over (L)}) and the assembled replacement ({tilde over (R)}) respectively, wherein an assembly of elements of the first and second rewriting rules consists, for each element, of a gathering of constituents of the element of the first rewriting rule with constituents of the element of the second rewriting rule; computing a merged graph (Ã) by applying the third double push-out graph rewriting rule to the initial graph (A), wherein the merged graph is translated in a merged modeled object.