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+---
+title: Licence arithmetic
+...
+
+Combining code from different sources in a free or open source
+software project requires making sure that the licences of each
+component are compatible. As an example, the GNU General Public
+Licence version 2 is incompabible with the Apache Licence version 2.
+However, GPLv3 is compatible with it.
+
+Checking things manually is tedious and error prone. For the most
+common licences compatibilities are pretty clear, and when each
+component declares its licences, a program can check the easy cases.
+
+Idea: Write a program that can do "licence arithmetic", by checking if
+combinations of licences are OK. The program would be given a "licence
+expression" giving the licences of its components, and outputs the
+effective licence, or "error", or "unknown".
+
+Use SPDX licence expressions as input and output.
+See <https://spdx.org/licenses/> for a list of licence identifiers.
+Assume AND and OR operators, and parentheses.
+
+Examples:
+
+* "GPL-2.0-only AND Apache-2" => "error"
+  * the licenses are known to be incompatible, so the result is not
+    free software; using it may be OK, but the combination is not
+    re-distributable
+* "GPL-2.0-or-later AND Apache-2" => "GPL-3.0-or-later"
+  * version 3 of the GPL is, however, compatible with Apache licence
+    version 2, so using the "or later" of the GPL'd component results
+    in an effective licence of GPL version 3 or later
+* "unknown AND BSD-2-Clause" = "error"
+  * a component whose licence is unknown cannot be combined with
+    anything; the result may be useable locally, but it's not
+    distributable
+* "other AND BSD-2-Clause" = "unknown"
+  * other means the licence is one that SPDX does not have a symbol
+    for; the result is unknown
+
+The program should probably read a data file that explains the known
+cases. Something like this:
+
+~~~yaml
+rules:
+  - expr: GPL-2.0-only AND BSD-2-Clause
+    result: GPL-2.0-only
+
+  - expr: GPL-3.0-or-later AND (BSD-2-Clause OR Apache-2)
+    result: GPL-3.0-or-later
+    
+  - expr: GPL-2.0-or-later AND Apache-2
+    result: GPL-3.0-or-later
+
+  - expr: GPL-2.0-only AND Apache-2
+    result: error
+    url: https://url.to.explanation
+    explanation: "explanation of why the result"
+~~~
+
+The program would extract the SPDX licence expressions of all
+components, combine them with OR, and simplify the result, then check
+against it against the rules in its data file.
+
+# Consult a lawyer
+
+Software can't give legal advice. It should only be used in entirely
+unambiguous cases and even then the result should be checked by a
+competent human.