6 rules in revocation of tax exemption

Below are six (6) basic rules to be kept in mind when facing questions regarding the revocation of tax exemptions.

[1] It cannot impair the obligation of contracts which is guaranteed by the Constitution.
[2] If granted with consent, it must be altered or revoked with consent.
[3] If granted by the Constitution, revocation must be done via amendment or revision.
[4] If granted by a special law, a general law using general terms of revocation cannot be construed as revoking the exemption under the special law.
[5] For a general law using general terms to revoke an exemption granted by special law, there must be manifest intent to repeal or alter the latter. An example of this would be the Local Government Code on the tax exemption of GOCCs.
[6] If withdrawn, exemption can still be granted in the future.