--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/+util/tt2t.m	Thu Sep 17 10:12:50 2015 +0200
@@ -0,0 +1,28 @@
+% Converts a semidiscretized PDE from second order in time to first order in time.
+%   v_tt = Dv + S
+% becomes
+%   w_t = Mw + C
+% where
+%   w = [ v ; v_t]
+% and
+%   M = [0 I;
+%        D 0];
+%   C = [0;
+%        S];
+function [M,C] = tt2t(D,S)
+    default_arg('S',sparse(size(D)))
+    time_dependent_bc = isa(S,'function_handle');
+
+    I = eye(size(D));
+    O = zeros(size(D));
+
+    M = [O I;
+         D O];
+
+    if ~time_dependent_bc
+        C = [zeros(size(S)); S];
+    else
+        o = zeros(size(S(0)));
+        C = @(t)([o; S(t)]);
+    end
+end