mal/ps/env.ps

% outer binds exprs -> env_new -> new_env
/env_new { 3 dict begin
    %(in env_new\n) print
    /exprs exch dup _sequential? { /data get }{ pop [ ] } ifelse def
    /binds exch dup _sequential? { /data get }{ pop [ ] } ifelse def
    /outer exch def
    << 
        /__outer__ outer
        0 1 binds length 1 sub {
            /idx exch def
            binds idx get (&) eq { %if &
                binds idx 1 add get % key
                exprs idx   exprs length idx sub   getinterval % value
                _list_from_array
                exit
            } if
            binds idx get % key
            exprs idx get % value
        } for
    >>
end } def

% env key -> env_find -> env
% env key -> env_find -> null
/env_find { 2 dict begin
    /key exch def
    /env exch def
    env key known { %if key in env
        env
    }{ env /__outer__ get null ne { %elseif __outer__ not null
        env /__outer__ get   key env_find
    }{ %else
        null
    } ifelse } ifelse
end } def

% env key val -> env_set -> val
/env_set {
    dup 4 1 roll % stack: val env key val
    put % stack: val
} def

/env_get {
    dup 3 1 roll % stack: key env key
    env_find % stack: key env/null
    dup null eq {
        pop % stack: key
        (') exch % stack: (') key
        dup length string cvs
        (' not found)
        concatenate concatenate
        _throw
    }{
        exch % stack: env key
        get
    } ifelse
} def