BUR #21051 has been approved by @nonamethanks.
create alias half-life -> half-life_(series)
create alias black_op -> black_op_(half-life)
create alias opposing_force -> half-life:_opposing_force
create alias strider -> strider_(half-life)
create implication half-life_1 -> half-life_(series)
create implication half-life:_decay -> half-life_(series)
create implication half-life:_blue_shift -> half-life_(series)
create implication half-life:_opposing_force -> half-life_(series)
create implication half-life:_alyx -> half-life_(series)
Half-Life is the first game in the series, and this will disambiguate the tag for use as the umbrella half-life tag.
Half-Life 1 and Half-Life: Alyx are entries in the Half-Life series.
[EDIT] Opposing Force should have Half-Life in it's title, as it is an entry in the Half-Life series.
[EDIT 2] Black Op is too vague for a character tag. Blue Shift and Decay are entries in the Half-Life series.
[EDIT 3] Strider is too vague for a character tag.
Updated