Notes to assist in debugging pmie

pmie command line debug flags
    APPL0	- lexical scanning
    APPL1	- parse/expression tree construction
    APPL2	- expression execution

macro EVALARG(x) expands to
    if ((x)->op < NOP) ((x)->eval)(x);

The source file fun.c is generated by expansion of all of the *.sk
"skeletal" files ... so changes need to be made in the *.sk files and
fun.c will be recreated in the make.

The Expr structure (defined in dstruct.h) is at the core of the pmie
expression evaluation.
    valid	must be > 0 for there to be any values able to be
    		used in the expression evaluation

