presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/getUnboundedFrameBoundType
class AstBuilder:    private static FrameBound.Type getUnboundedFrameBoundType(Token token)
    {
        switch (token.getType()) {
            case SqlBaseLexer.PRECEDING:
                return FrameBound.Type.UNBOUNDED_PRECEDING;
            case SqlBaseLexer.FOLLOWING:
                return FrameBound.Type.UNBOUNDED_FOLLOWING;
        }

        throw new IllegalArgumentException("Unsupported bound type: " + token.getText());
    }