We are to / we are not to
Henrique A da Silva from Brazil writes:
  I've been living in the US for a few months and I've had opportunity to read some articles and books more easily. But I've had doubts about some constructions. I wonder how it is that to is used in these sentences:

PM to meet Espada today
Spanish mob gathers at border ?UN to mediate

Also, do the words mob and crowd have the same meaning and usage?
 
 
Roger Woodham replies:
 
 
he is to / they are to
 
 We use this structure quite a lot to talk about official plans and arrangements. In your examples, Enrique, the verb be is omitted which is very characteristic of newspaper headlines. In any text follow the headline it would need to be included, e.g.
 
The British Prime Minister, the French President and the German Chancellor are to meet in Berlin this weekend.
 
Note that we can also use this structure as a perfect infinitive to describe a planned event that did not take place:
 
The British PM was to have had talks with his Canadian counterpart on his way to the US, but these were cancelled when the trip was curtailed. I was to have gone on holiday with Ruth, but she couldn't get time off work.
 
In a more informal register, was supposed to has a similar meaning:
 
I was supposed to go on holiday with Ruth, but she couldn't get time off work.
 
you are not to ? I am to
 
 Note that this structure can also be used to issue or to acknowledge instructions or orders. When it is used in negative sentences, it nearly always has this meaning, but it can be used in affirmative sentences with this meaning too
 
We are to smoke only in the designated areas. We are not to smoke anywhere else.
 
We are to look after our neighbours?dog while they are away but we’re not to feed him any chocolate. Else he will be sick.
 
There is future meaning here, but it also means at any time, particularly in negative sentences. As we are acknowledging an instruction, we could also use we mustn't in these sentences instead of we’re not to.
 
Here are some more examples of this usage. Look at these instructions to members of a youth club in an inner-city area:
 
1 Respect yourself and other members.
2 No scrapping or fighting and no weapons.
3 No bullying, cussing or dissing.
4 No setting off fire alarms unless there is fire.
5 No climbing on the roof.
6 Entrance through front door only, not back door.
 
We can re-phrase these instructions, using are to / are not to. We can also use be permitted to or be allowed to as an alternative to mustn't as the obligation is imposed externally. (I have also tried to paraphrase youth-culture vocabulary in this re-wording):
 
1 You are to respect yourself and other members at all times.
2 There is to be no scrapping or fighting and no weapons of any kind are permitted.
3 You mustn't bully anyone and you are not allowed to swear at anyone or insult other members.
4 Fire alarms are not to be set off unless there is fire.
5 Climbing on the roof is not permitted.
6 You are to enter the youth club only through the front door, not through the back entrance.
 
mob / crowd
 
 A mob is a large crowd of people that is difficult to control. It often collocates with the adjectives angry or unruly. (Unruly = undisciplined):
 
An unruly mob surrounded the parliament building.
 
Crowd is more neutral and simply describes a large gathering of people.
Crowds can be peaceful or angry:
 
He is a brilliant orator and attracted a crowd of 5000 to Speaker's Corner in Hyde Park.
An angry crowd had gathered outside the police station.
 
 
    
Crowd