Parses the tokens of Otera.Lexer into an abstract syntax tree, with the grammar of the Python Jinja2 parser.

The tree is a list of nodes. Nodes are tuples, so that the evaluator can pattern match on them.

Statements:

• {:text, string} and {:output, expr}
• {:if, [{test, body}], else_body}
• {:for, target, iter, filter_or_nil, body, else_body, recursive?}
• {:set, target, expr} and {:set_block, target, filter_or_nil, body}
• {:macro, name, params, body, special_names} and {:call_block, call, params, body, special_names}
• {:filter_block, filter, body}, {:with, [{target, expr}], body} and {:generation, body}
• :break and :continue

Expressions:

• {:const, value}, {:name, name}, {:list, items}, {:tuple, items} and {:dict, [{key, value}]}
• {:getattr, expr, name}, {:getitem, expr, key} and {:slice, start, stop, step} as a key
• {:call, callee, args, kwargs}, {:filter, name, expr, args, kwargs, line} and {:test, name, expr, args, kwargs, line}, where args has expressions and {:unpack, expr} for *expr, and kwargs has {name, expr} and {:unpack, expr} for **expr
• {:not, expr}, {:neg, expr}, {:pos, expr}, {:and, left, right} and {:or, left, right}
• {:binop, op, left, right} with op :add, :sub, :mul, :div, :floordiv, :mod or :pow
• {:concat, exprs}, {:compare, expr, [{op, expr}]} and {:cond, test, then, else}
• {:ns_ref, name, attr} as an assignment target

A filter of a filter block or a set block has nil as its innermost expression. An inline if without else gets an undefined value as its else. params of a macro is a list of {name, default_or_nil}, and special_names lists which of caller, varargs and kwargs the body uses.

As in Jinja2, an unknown filter or test is a syntax error, except inside an if block or an inline if, where it fails only when the template uses it.