add references to README

Author: igorw

README.md

@@ -112,3 +112,13 @@ Or `r` for raw mode:
 * [Matt Might: 7 lines of code, 3 minutes](http://matt.might.net/articles/implementing-a-programming-language/)
 * [Tom Stuart: Programming with Nothing](http://codon.com/programming-with-nothing)
 * [Erkki Lindpere: Parsing Lambda Calculus in Scala](http://zeroturnaround.com/rebellabs/parsing-lambda-calculus-in-scala/)
+
+## References
+
+* [Jean-Louis Krivine: A call-by-name lambda-calculus machine](http://www.pps.univ-paris-diderot.fr/~krivine/articles/lazymach.pdf)
+* [Rémi Douence, Pascal Fradet: The Next 700 Krivine Machines](http://pop-art.inrialpes.fr/~fradet/PDFs/HOSC07.pdf)
+* [Xavier Leroy: The Zinc Experiment](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.43.6772)
+* [John Tromp: Binary Lambda Calculus and Combinatory Logic](http://homepages.cwi.nl/~tromp/cl/LC.pdf)
+* [John Tromp: Binary Lambda Calculus interpreter for IOCCC](http://www.ioccc.org/2012/tromp/hint.html)
+* [Binary Lambda Calculus in Python](https://github.com/sdiehl/bnlc)
+* [Krivine Machine in Scheme](https://github.com/ympbyc/Carrot)

src/krivine.php

@@ -155,7 +155,7 @@ function push($args, $code, $env, $stack)
     function grab($args, $code, $env, $stack)
     {
         list($code0, $env0) = first($stack);
-        $code0 = $code0 ?: [['unclosed_term']];
+        $code0 = $code0 ?: [];
         $env0 = $env0 ?: [];
 
         array_unshift($env, [$code0, $env0]);