# MUST

Creates an obligation for a party to perform an action. The party is required to do something.

## Syntax

```l4
PARTY partyName MUST DO action
PARTY partyName MUST DO action WITHIN deadline
PARTY partyName MUST NOT DO action
```

## Purpose

MUST expresses a legal obligation - something that a party is required to do. If not fulfilled within the deadline, consequences (LEST) may apply.

## Examples

**Example file:** 

```l4-file
-- MUST Keyword Examples - Legal Obligations

-- Type Declarations

DECLARE `Contract Party` IS ONE OF
  `The Borrower`
  `The Lender`
  `The Buyer`
  `The Seller`
  `The Contractor`
  `The Client`
  `The Tenant`
  `The Landlord`

DECLARE `Contract Action` IS ONE OF
  `make payment` HAS `payment amount` IS A NUMBER
  `deliver goods` HAS `goods description` IS A STRING
  `provide services` HAS `service description` IS A STRING
  `sign contract`
  `provide notice` HAS `notice content` IS A STRING
  `complete repairs` HAS `repair description` IS A STRING
  `pay penalty` HAS `penalty amount` IS A NUMBER
  `return security deposit` HAS `deposit amount` IS A NUMBER

-- Basic Payment Obligation

-- Simple payment obligation with a deadline
-- The Borrower must make a payment of $10,000 within 30 days
`monthly loan payment` MEANS
  PARTY `The Borrower`
  MUST `make payment` 10000
  WITHIN 30

-- With HENCE (Consequence on Fulfillment)

-- Purchase contract: If The Buyer pays, then The Seller must deliver
-- This creates a conditional chain of obligations
`purchase and delivery agreement` MEANS
  PARTY `The Buyer`
  MUST `make payment` 25000
  WITHIN 7
  HENCE
    PARTY `The Seller`
    MUST `deliver goods` "industrial machinery"
    WITHIN 14

-- Service contract: After payment, contractor provides services,
-- then client pays the remaining balance
`staged service contract` MEANS
  PARTY `The Client`
  MUST `make payment` 5000  -- Initial deposit
  WITHIN 3
  HENCE
    PARTY `The Contractor`
    MUST `provide services` "website development"
    WITHIN 30
    HENCE
      PARTY `The Client`
      MUST `make payment` 15000  -- Final payment
      WITHIN 7

-- With LEST (Consequence on Breach)

-- Simple breach: If The Borrower doesn't pay, the contract is breached
`payment or breach` MEANS
  PARTY `The Borrower`
  MUST `make payment` 5000
  WITHIN 30
  LEST BREACH

-- Penalty clause: If payment not made on time, a late fee applies
-- with a grace period before final breach
`payment with late fee` MEANS
  PARTY `The Borrower`
  MUST `make payment` 10000
  WITHIN 30
  LEST
    PARTY `The Borrower`
    MUST `make payment` 12000  -- Original amount plus 20% late fee
    WITHIN 45  -- 15-day grace period
    LEST BREACH

-- Landlord repair obligation: Must repair within deadline,
-- or tenant can deduct repair costs from rent
`landlord repair obligation` MEANS
  PARTY `The Landlord`
  MUST `complete repairs` "fix leaking roof"
  WITHIN 14
  LEST
    PARTY `The Tenant`
    MUST `provide notice` "tenant will arrange repairs and deduct cost"
    WITHIN 3

-- MUST NOT (Prohibition)

-- MUST NOT is equivalent to SHANT - expresses a prohibition
-- The Contractor must not disclose confidential information
-- for 2 years (730 days)
`confidentiality obligation` MEANS
  PARTY `The Contractor`
  MUST NOT `provide notice` "confidential information disclosure"
  WITHIN 730

-- Chained Obligations

-- Complex contract with multiple sequential obligations
-- Demonstrates a complete contract lifecycle
`construction contract workflow` MEANS
  PARTY `The Contractor`
  MUST `sign contract`
  WITHIN 7
  HENCE
    PARTY `The Client`
    MUST `make payment` 50000  -- Down payment
    WITHIN 14
    HENCE
      PARTY `The Contractor`
      MUST `provide services` "foundation work"
      WITHIN 30
      HENCE
        PARTY `The Client`
        MUST `make payment` 150000  -- Final payment
        WITHIN 7

-- Lease agreement with security deposit return
`lease termination and deposit return` MEANS
  PARTY `The Tenant`
  MUST `provide notice` "60-day notice of non-renewal"
  WITHIN 60
  HENCE
    PARTY `The Tenant`
    MUST `complete repairs` "restore premises to original condition"
    WITHIN 30
    HENCE
      PARTY `The Landlord`
      MUST `return security deposit` 3000
      WITHIN 14
      LEST
        PARTY `The Landlord`
        MUST `return security deposit` 3090  -- With 3% interest penalty
        WITHIN 30
        LEST BREACH

-- Testing Obligations

-- Test 1: Basic obligation fulfilled on time
#TRACE `monthly loan payment` AT 0 WITH
  PARTY `The Borrower` DOES `make payment` 10000 AT 20

-- Test 2: Basic obligation breached (no payment made)
#TRACE `monthly loan payment` AT 0 WITH

-- Test 3: Conditional obligation - full chain executed
#TRACE `purchase and delivery agreement` AT 0 WITH
  PARTY `The Buyer` DOES `make payment` 25000 AT 5
  PARTY `The Seller` DOES `deliver goods` "industrial machinery" AT 15

-- Test 4: Payment with late fee - paid during grace period
#TRACE `payment with late fee` AT 0 WITH
  PARTY `The Borrower` DOES `make payment` 12000 AT 40

-- Test 5: Payment with late fee - breached (no payment at all)
#TRACE `payment with late fee` AT 0 WITH

-- Test 6: Staged service contract - complete workflow
#TRACE `staged service contract` AT 0 WITH
  PARTY `The Client` DOES `make payment` 5000 AT 2
  PARTY `The Contractor` DOES `provide services` "website development" AT 25
  PARTY `The Client` DOES `make payment` 15000 AT 30

-- Test 7: Construction workflow - happy path
#TRACE `construction contract workflow` AT 0 WITH
  PARTY `The Contractor` DOES `sign contract` AT 5
  PARTY `The Client` DOES `make payment` 50000 AT 15
  PARTY `The Contractor` DOES `provide services` "foundation work" AT 40
  PARTY `The Client` DOES `make payment` 150000 AT 45

-- Test 8: Lease termination with deposit return
#TRACE `lease termination and deposit return` AT 0 WITH
  PARTY `The Tenant` DOES `provide notice` "60-day notice of non-renewal" AT 30
  PARTY `The Tenant` DOES `complete repairs` "restore premises to original condition" AT 55
  PARTY `The Landlord` DOES `return security deposit` 3000 AT 65

-- Test 9: Landlord repair obligation - landlord complies
#TRACE `landlord repair obligation` AT 0 WITH
  PARTY `The Landlord` DOES `complete repairs` "fix leaking roof" AT 10

-- Test 10: Landlord repair obligation - tenant must notify
#TRACE `landlord repair obligation` AT 0 WITH
```



## Obligation Fulfillment

- Obligation is **fulfilled** when the party performs the action before the deadline
- Obligation is **breached** when the deadline passes without the action
- Consequences in LEST clause activate on breach

## Related Keywords

- **[PARTY](/l4/reference/regulative/PARTY.md)** - Identifies who has the obligation
- **[MAY](/l4/reference/regulative/MAY.md)** - Permission (optional action)
- **[SHANT](/l4/reference/regulative/SHANT.md)** - Prohibition
- **[REGULATIVE](/l4/reference/regulative.md)** - Full regulative rule reference