[lmm]

It is visibly worse than python. '|' - I thought that was bitwise or? - rather than an (admittedly technical) English word like 'map' or 'filter' (and '<<' has the same problem). 'transformed([](pv){...})' may be necessary for compatibility, but the brackets are harder to follow than 'transformed(lambda pv: ...)'. And the type declaration as "propMap::value_type" is just noise - it doesn't tell the reader anything about what the actual type is, and it isn't necessary for static typechecking either (it would be optional in a modern statically typed language e.g. Scala).