Dropping locants

Basic IUPAC Organic Nomenclature

When can you drop the locant ?

According to the IUPAC rules, locants can be omitted if:

omitting the locants gives an unambiguous structure,
the parent system contains only one type of substitutable H,
all substitutable H (C-H) have been replaced by the same substituent,
all substituted H (C-H) have the same unambiguous location (and hence locants)
the principal functional group must be at the end of the chain (e.g. aldehydes, carboxylic acids, nitriles etc.)

(note that some of these conditions overlap with each other).

Here are some illustrative examples that try to capture these implications of this at this "introductory organic chemistry" level:

ethanol	Locant dropped	unambiguous, all C atoms in the parent chain (ethane) are equivalent
propene	Locant dropped	unambiguous, C=C must be at the end of the parent chain
propan-1-ol	Locant can not be dropped	ambiguous, -OH group could be on C1 or C2 of the parent chain (propane) which are different
propanal	Locant dropped	unambiguous, an aldehyde must be at the end of the chain which defines C1
propan-2-one	Locant can not be dropped	Ketones aren't restricted to ends of chains and the parent chain (propane) has two different positions. (IUPAC specifically cites this example)
cyclohexanol	Locant dropped	unambiguous, all positions on the parent ring (cyclohexane) are equivalent
butan-2-one	Locant can not be dropped	Ketones aren't restricted to ends of chains and the parent chain (butane) has two different positions (IUPAC specifically cites this example)
2-chloroethan-1-ol	Locants can not be dropped	View ethanol as the parent, then there are two places the substituent could be and therefore required to include both locants. (IUPAC specifically cites this example)
cyclohexane-1,2-diol	Locants can not be dropped	With two groups on a cyclic structure both locants are required according to the IUPAC rules to distinguish 1,1-, 1,2-, 1,3- etc.
prop-2-en-1-ol	Locants can not be dropped	With two groups on parent structure both locants are required according to the IUPAC rules to distinguish 1,1- & 1,2-.
cyclohex-2-en-1-ol	Locants can not be dropped	With two groups on a cyclic structure both locants are required according to the IUPAC rules to distinguish 1,1-, 1,3-, 1,4- etc.
3-bromocyclohex-1-ene	Locants can not be dropped	With two groups on a cyclic structure both locants are required according to the IUPAC rules to distinguish 1,1-, 1,3-, 1,4- etc. (IUPAC specifically cites this example)