In shells that support extended assignment syntax, for example to allow an array to be populated with a single assignment, such extensions can typically only be used in assignments specified as arguments to export if the command word is literally export, and not if it is some other word that expands to export. For example:

# Shells that support array assignment as an extension generally support this:
export x=(1 2 3); echo ${x[0]}  # outputs 1
# But generally do not support this:
e=export; $e x=(1 2 3); echo ${x[0]}  # syntax error

Some implementations extend the shell's assignment syntax, for example to allow an array to be populated with a single assignment, and in order for such an extension to be usable in assignments specified as arguments to export these shells have export as a separate token in their grammar. This standard only permits an extension of this nature when the input to the shell would contain a syntax error according to the standard grammar. Note that although export can be a separate token in the shell's grammar, it cannot be a reserved word since export is a candidate for alias substitution whereas reserved words are not (see [xref to 2.3.1]).

None.

In shells that support extended assignment syntax, for example to allow an array to be populated with a single assignment, such extensions can typically only be used in assignments specified as arguments to readonly if the command word is literally readonly, and not if it is some other word that expands to readonly. For example:

# Shells that support array assignment as an extension generally support this:
readonly x=(1 2 3); echo ${x[0]}  # outputs 1
# But generally do not support this:
r=readonly; $r x=(1 2 3); echo ${x[0]}  # syntax error

Some implementations extend the shell's assignment syntax, for example to allow an array to be populated with a single assignment, and in order for such an extension to be usable in assignments specified as arguments to readonly these shells have readonly as a separate token in their grammar. This standard only permits an extension of this nature when the input to the shell would contain a syntax error according to the standard grammar. Note that although readonly can be a separate token in the shell's grammar, it cannot be a reserved word since readonly is a candidate for alias substitution whereas reserved words are not (see [xref to 2.3.1]).

... or that prevents function lookup on b or c.

... or that prevents function lookup on b or c. However, some implementations extend the shell's assignment syntax, for example to allow an array to be populated with a single assignment, and in order for such an extension to be usable in assignments specified as arguments to export and readonly these shells have those utility names as separate tokens in their grammar. When command is used to execute these utilities it also needs to be a separate token in the grammar so that the same extended assignment syntax can still be recognized in this case. This standard only permits an extension of this nature when the input to the shell would contain a syntax error according to the standard grammar, and therefore it cannot affect how '|' and ';' are parsed in the example above. Note that although command can be a separate token in the shell's grammar, it cannot be a reserved word since command is a candidate for alias substitution whereas reserved words are not (see [xref to 2.3.1]).