eval method
Implementation
dynamic eval( {String key : "",Expression? exp})
{
late Expression torun;
if(exp != null) { torun = exp; }
else if(expressions.containsKey(key)) { torun = expressions[key];}
else {
//print("neither $key nor $exp bailing");
return(-1);
}
if(torun != null) {
final evaluator = const ExpressionEvaluator();
var r = evaluator.eval(torun, vars);
print("evaled result = '$r'");
return(r);
}
return(-1);
}