The Alan source code is freely available under the Artistic License 2.0 with the intention to

• freely distribute games produced with the Alan system, including for profit
• re-distribute compiled versions of the Alan system, including together with a game which is not open source or free, provided there is no charge for the Alan system
• redistribute compiled and/or source versions of the original Alan system (the Standard Version)
• aquire the source code for the Standard Version
• modify the source code for private use
• re-distribute compiled and/or source of a Modified Version provided they are done so under a compatible license with appropriate attribution *and* that the modification is discribed and made available, preferably by returning it to the Copyright Holder so that it can be merged into the Standard Version

You can find the source code on GItHub.