After so many years of searching, finally, I found Piptospatha impolita in a sandstone stream in western Sarawak. It is endemic to the streams in that small area only...hence it is a very rare aroid from Sarawak!!!
Aridarum borneensis (right) & Piptospatha impolita (left).
Ginger, Camptandra sp. grows on a boulder.
Flowers of a jungle palm.