AST objects may be created from source code or from a parse tree.
When creating an AST object from source, different functions are used
to create the 'eval' and 'exec' forms.
del f(0), which escapes the Python parser but is
checked by the bytecode compiler.
Sequences representing terminal tokens may be represented as either
two-element lists of the form (1, 'name') or as three-element
lists of the form (1, 'name', 56). If the third element is
present, it is assumed to be a valid line number. The line number
may be specified for any subset of the terminal symbols in the input
tree.