<s>
It	O
is	O
based	O
on	O
the	O
calculus	B-Application
of	I-Application
constructions	I-Application
with	O
inductive	O
types	O
.	O
</s>
<s>
The	O
Lean	O
project	O
is	O
an	O
open-source	B-License
project	O
hosted	O
on	O
GitHub	B-Application
.	O
</s>
<s>
Lean	O
has	O
an	O
interface	O
,	O
implemented	O
as	O
a	O
Visual	B-Language
Studio	I-Language
Code	I-Language
extension	O
and	O
Language	O
Server	O
Protocol	O
server	O
,	O
that	O
differentiates	O
it	O
from	O
other	O
interactive	O
theorem	O
provers	O
.	O
</s>
<s>
It	O
has	O
native	O
support	O
for	O
Unicode	O
symbols	O
,	O
which	O
can	O
be	O
typed	O
using	O
LaTeX-like	O
sequences	O
,	O
such	O
as	O
"	O
\times	O
 "	O
for	O
"	O
×	O
"	O
.	O
</s>
<s>
Lean	O
can	O
also	O
be	O
compiled	O
to	O
JavaScript	B-Language
and	O
accessed	O
in	O
a	O
web	O
browser	O
and	O
has	O
extensive	O
support	O
for	O
meta-programming	B-Application
.	O
</s>
<s>
A	O
user	O
wishing	O
to	O
change	O
these	O
parts	O
of	O
the	O
system	O
would	O
need	O
to	O
modify	O
the	O
C++	B-Language
implementation	O
of	O
Lean	O
.	O
</s>
<s>
Furthermore	O
,	O
overhead	O
associated	O
with	O
virtual	O
machine	O
interpretation	O
meant	O
that	O
the	O
efficiency	O
of	O
Lean	O
was	O
not	O
competitive	O
with	O
other	O
proof	O
assistants	O
such	O
as	O
Coq	B-Application
.	O
</s>
<s>
In	O
2021	O
,	O
Leonardo	O
de	O
Moura	O
and	O
Sebastian	O
Ullrich	O
released	O
Lean	O
4	O
:	O
a	O
reimplementation	O
of	O
the	O
Lean	B-Language
theorem	I-Language
prover	I-Language
within	O
itself	O
,	O
with	O
the	O
aim	O
of	O
addressing	O
the	O
two	O
aforementioned	O
criticisms	O
.	O
</s>
<s>
Lean	O
4	O
produces	O
C	B-Language
code	O
which	O
is	O
then	O
compiled	O
,	O
enabling	O
the	O
development	O
of	O
efficient	O
domain-specific	O
automation	O
.	O
</s>
<s>
Lean	O
4	O
is	O
not	O
backwards-compatible	B-General_Concept
with	O
Lean	O
3	O
.	O
</s>
<s>
Addition	O
of	O
natural	O
numbers	O
can	O
be	O
defined	O
recursively	O
,	O
using	O
pattern	B-Language
matching	I-Language
.	O
</s>
