“…Later developments focused mainly on understanding WKB-theoretic transformation series (first introduced in [AKT91]) that transform a given differential equation in a suitable neighbourhood of critical WKB trajectories (or Stokes lines) to one in standard form whose WKB-theoretic properties are better understood. A very partial list of contributions includes the works by Aoki, Kamimoto, Kawai, Koike, Sasaki, and Takei, [AKT09, KK11, KKT12, KK13,Sas13,KKT14a]. Parallel to this activity has been the classification of WKB geometry (or Stokes graphs), which includes the works by Aoki, Kawai, Takei, Tanda, [AKT01, Tak07, AT13, Tak17], as well as a detailed analysis of some WKB-theoretic properties of special classes of equations, which includes the works by Aoki, Kamimoto, Kawai, Koike, and Takei [KT11, KKKT11, ATT14, KKT14b, Tan15, KKK16, ATT16, AT16, AIT19].…”