Fprint "pretty-prints" an AST node to output.

do not use a tabwriter; if set, UseSpaces is ignored.

emit //line directives to preserve original source positions.

use tabs for indentation independent of UseSpaces.

use spaces instead of tabs for alignment.

A CommentedNode bundles an AST node and corresponding comments.

A Config node controls the output of Fprint.

A Mode value is a set of flags (or 0).