\[ \left \{x'(t)=a y(t),y'(t)=-a x(t)\right \} \] ✓ Mathematica : cpu = 0.009591 (sec), leaf count = 39
\[\{\{x(t)\to c_1 \cos (a t)+c_2 \sin (a t),y(t)\to c_2 \cos (a t)-c_1 \sin (a t)\}\}\] ✓ Maple : cpu = 0.042 (sec), leaf count = 35
\[ \left \{ \left \{ x \left ( t \right ) ={\it \_C1}\,\sin \left ( at \right ) +{\it \_C2}\,\cos \left ( at \right ) ,y \left ( t \right ) =\cos \left ( at \right ) {\it \_C1}-\sin \left ( at \right ) {\it \_C2} \right \} \right \} \]