MC57: Assemble a set of finite-element matrices

This subroutine assembles a set of element matrices, that is, it forms the summation

\[\mathbf{A} = \sum_ l \mathbf{A} ^{[l]},\] where each element matrix \(\mathbf{A} ^{[l]}\) has entries only in the principal submatrix corresponding to the variables in element \(l\). Each \(\mathbf{A} ^{[l]}\) must be held in packed form as a small full square matrix, together with a list of the variables associated with element \(l\). The assembled matrix \(\mathbf{A}\) has a symmetric sparsity pattern but may be unsymmetric. An option exists for assembling only the sparsity pattern of \(\mathbf{A}\). If the variables are not indexed contiguously, absent rows and columns may optionally be removed.