easy to understand and modify but not meant for speed
following Smears, I., Vohralík, M. Simple and robust equilibrated flux a
posteriori estimates for singularly perturbed reaction–diffusion problems.
ESAIM Math. Model. Numer. Anal. (2020), DOI 10.1051/m2an/2020034, Journal version, preprint .
a simple example scriptRD_simple.edp the full script used for the numerical experimentsRD_full.edp