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2) Types and Variables
Any string of uppercase latin letters is a variable identifier, initially not unified to anything. These variables and their possible values are not shared between rules.
? and . are special variables which are the Input and the Output of the current rule they are in.
The input => This has been reverted due to a bug? can take subscripts (like built-in predicates). ?₍ is the first element of the Input. ?₎ is the last element of the Input. ?₄₂ for example is the 42th element of the Input.
Variables can be unified by using them as inputs or outputs to predicates or by direcly unifying them with other variables. For example, I J will unify I with J (or fail if they cannot be unified).
Lists are noted in between square brackets [...] and elements of a list are separated using ,. One can also construct a list using :. For example, :2:3. will unify the Output with the list [Input,2,3]. However, 2:3 will not unify the Input with the list [2,3] ; it will instead unify the Input with 2 and then construct the list [2,3] ([2,3] would have unified the Input with [2,3]).
[] denotes the empty list.
Strings are opened and closed with double quotes "...". \ will escape double quotes.
Integers and floats are written using the numbers 0 to 9 as expected. The decimal separator is .. A floating point number must have at least one digit before and after the decimal separator.
Negative numbers are written using an underscore _ preceeding the number. This underscore is called the low minus in Brachylog.