Love Picol, and love this! When I first revisited Tcl, I was a bit miffed about needing [expr] but now really appreciate both it and the normal Tcl syntax.
I have a Tcl Improvement Proposal (TIP 676) currently being voted on which introduces an alternative compact form of calculation. The implementation uses a Pratt parser: https://core.tcl-lang.org/tcl/file?ci=cgm-equals-command&nam... which directly generates bytecode rather than creating a parse tree.