Search Syntax Tree node to find variable names that are global.

def find_all_globals(node, globs):
    """Search Syntax Tree node to find variable names that are global."""
    for n in node:
        if isinstance(n, SyntaxTree):
            globs = find_all_globals(n, globs)
        elif n.kind in read_write_global_ops:
            globs.add(n.pattr)
    return globs