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===Syntax and semantics===
* ''Program Structure:'' A Janus program consists of global variable declarations followed by procedure declarations. The execution starts at a procedure named <code>main</code>, or the last procedure defined if <code>main</code> is absent.
* ''Data Types:'' Janus primarily uses 32-bit integers (interpreters may differ on signed vs. unsigned) and one-dimensional integer arrays of fixed size.<ref>{{cite conference
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** '''Swap:''' <code>x <=> y</code> exchanges the values of <code>x</code> and <code>y</code>.
** '''Conditional:''' <code>if e1 then s1 else s2 fi e2</code>. The expression <code>e1</code> is the test evaluated upon forward entry. The expression <code>e2</code> is an assertion evaluated upon forward exit; it must be true if <code>s1</code> was executed and false if <code>s2</code> was executed. For backward execution (e.g., via <code>uncall</code>), <code>e2</code> acts as the test to determine which inverse branch (s1<sup>−1</sup> or s2<sup>−1</sup>) to take, and <code>e1</code> becomes the assertion checked upon exiting backward.<ref>{{cite arXiv
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** '''Procedure Call:''' <code>call id</code> executes procedure <code>id</code> forwards; <code>uncall id</code> executes procedure <code>id</code> backwards.<ref name="Choudhury"/> Procedures operate via side effects on the global store.
** '''Skip:''' <code>skip</code> does nothing and is its own inverse.
** '''Sequence:''' <code>s1; s2</code>. The inverse is <code>s2<sup>−1</sup>; s1<sup>−1</sup></code>.
===Implementations and code examples===
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