The wave tip was found as the intersection of two isolines u=v=0, by
linear interpolation between their crossings of the mesh ribs. The phase
of the spiral wave was computed as 
at the
wavetip, according to the definition given in Biktashev et al [10]. This was obtained via the derivatives 
and 
computed by central differences at the
corners of the mesh cell containing the tip and then bilinearly interpolated
to the tip point. The instantaneous frequency of wavetip rotation at time t
was accepted as the value of 
with the least T;SPMgt;0
satisfying the condition 
.