// same order of magnitude
// as the other elements of
// the matrix
+ //
+ // note that this
+ // also captures
+ // the special case
+ // that a dof is
+ // both constrained
+ // and fixed (this
+ // can happen for
+ // hanging nodes in
+ // 3d that also
+ // happen to be on
+ // the
+ // boundary). in
+ // that case,
+ // following the
+ // above program
+ // flow, it is
+ // realized that
+ // when
+ // distributing the
+ // row and column
+ // no elements of
+ // the matrix are
+ // actually touched
+ // if all the
+ // degrees of
+ // freedom to which
+ // this dof is
+ // constrained are
+ // also constrained
+ // (the usual case
+ // with hanging
+ // nodes in
+ // 3d). however, in
+ // the line below,
+ // we do actually
+ // do something
+ // with this dof
if (i == j)
global_matrix.add (local_dof_indices[i],
local_dof_indices[i],
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