1. A system comprising: means for obtaining a parent matrix, N M , the parent matrix being a first moment matrix of a parent model, the parent matrix having elements, n i,j M , i and j being indices of the parent matrix, the parent matrix having first-row elements, n 1,j M , the parent matrix further having diagonal elements n j,j M ; means for obtaining a child matrix, N S , the child matrix being a first moment matrix of a child model, the child model being a submodel within the parent model, the child matrix having elements, n k,l S , k and l being indices of the child matrix, the child matrix having first-row elements, n 1,k S , the child matrix further having diagonal elements, n k,k S ; means for determining an expanded matrix, N F , the expanded matrix being a first moment matrix of the child model instantiated within the parent model, the means for determining the expanded matrix comprising: means for calculating a first set of first-row elements, n 1,j F , for the expanded matrix, the first set of first row elements being calculated according to: 
 n 1,j F =n 1,j M ; means for calculating a second set of first-row elements, n 1,k F , for the expanded matrix, the second set of first row elements being calculated according to: 
 n 1,k F =n 1,S M n 1,k S ; means for calculating a first set of diagonal elements, n j,j F , for the expanded matrix, the first set of diagonal elements being calculated according to: 
 n j,j F =n j,j M ; and means for calculating a second set of diagonal elements, n k,k F , for the expanded matrix, the second set of diagonal elements being calculated according to: 
 n k,k F =n k,k S +( n S,S M −1)· n 1,k S .