The ship/vessel/boat carries/transports/takes/brings coal to (any port).
The second type of restriction is the restriction in introducing any additional components into the structure of a phraseological unit.
In a free word-group such changes can be made without affecting the general meaning of the utterance: This big ship is carrying a large cargo of coal to the port of Liverpool.
In the phraseological unit to carry coals to Newcastle no additional components can be introduced. Nor can one speak about the big white elephant (when using the white elephant in its phraseological sense) or about somebody having his heart in his brown boots.
Yet, such restrictions are less regular. In Vanity Fair by W. M. Thackeray the idiom to build a castle in the air is used in this way:
"While dressing for dinner, she built for herself a most magnificent castle in the air of which she was the mistress ..."
In fiction such variations of idioms created for stylistic purposes are not a rare thing. In oral speech phraseological units mostly preserve their traditional structures and resist the introduction of additional components.
The third type of structural restrictions in phraseological units is grammatical invariability. A typical mistake with students of English is to use the plural form of fault in the phraseological unit to find fault with somebody (e. g. The teacher always found faults with the boy). Though the plural form in this context is logically well-founded, it is a mistake in terms of the grammatical invariability of phraseological units >. A similar typical mistake often occurs in the unit from head to foot (e. g. From head to foot he was immaculately dressed). Students are apt to use the plural form of foot