# Formatting rules for Lightweight Text Assurance Case (LTAC) format
v1.0, 27.10.2025

1. LTAC is a text format for representing assurance case arguments regardless of the notation originally used. LTAC can be mapped to GSN, CAE and TRUST-IT notations. LTAC argument contains a set of elements, each presented in a seperate line.
2. LTAC defines its own argument element types that can be mapped to GSN, CAE and TRUST-IT. LTAC defines its own syntaxt descibed below which is different from the rules of GSN, TRUST-IT and CAE. Only the rules desribed below apply to LTAC.
3. The allowed element types are Claim, Strategy, Justification, Evidence, Context and Assumption. Each argument in LTAC should start with a claim.
4. A keyword 'Link' is used to cite already specified argument element
5. Each argument element is defined in a row in LTAC that begins with dash and a type keyword (or 'Link' keyword) followed optionally by an identifier, e.g.
  - Claim C1
  - Strategy S1
  - Justification J1
  - Evidence E1
  - Context X1
  - Assumption A1
  - Link C4
6. If an element is to be used more than once, it should be defined once only and other occurences should be defined as links. A Link entry is a citation of previously defined element (often evidence or context) without repeating it.
7. Identifiers are optional, but when specified, they should be unique within the assurance case argument except links. Links are citations of existing elements and they use existing identifiers.
  - Identifiers G1, G2 can be used for claims, or Cxt1, Cxt2 can be used for context elements. Any lettes, digits and other characters except ':' can be used in identifiers. Identifiers’ prefixes other than presented in point 1 are acceptable.
8. After the identifier, a colon (':') separates it from the descriptive text, the element name. The text cannot contain new line characters. The text can contain colons (':') . An example of a correct element definition:
```
  - Claim C1: System is sufficiently safe
```
9. Argument structure
  a) The assurance case argument is structured using indentation. Two spaces are added for each level of the argument structure.
  b) Child nodes (sub-claims, evidence, strategies, etc.) are indented under their parents.
  c) Each indentation level, two spaces, represents one step deeper in the argument hierarchy.
  d) Hyphens ‘-‘ are used at the start of each line to mark list items.
  e) Evidence references may cite external artifacts, written inside parentheses at the end of element name
  f) Justifications may need to be supported by evidence or an argument: subclaims or a strategy.
  g) The following relationships are permitted within the argument structure, which is presented in the format 'element -> sub-element'. Any relationships not listed below are not permitted.
    - Claim -> Claim, Strategy, Assumption, Justification, Evidence
    - Strategy -> Claim, Justification, Assumption
    - Justification -> Claim, Strategy, Evidence
    - Any element -> Context
  h) Claims supported by Evidence should not be supported by any Justifications, Assumptions nor Strategies.
    - Justifications and Assumptions should be used for higher-level claims and strategies.
	- If a Claim is to be supported by Evidence and by Strategy, the Evidence should be replaced by a Claim describing it and this Claim can be supported by this Evidence element
  i) LTAC can be used to present a single module argument, not modular arguments. Interfaces and away elements cannot be represented in LTAC. Other more advanced notations can be used to represent modular arguments.
  j) It's not allowed to add additional text to LTAC argumentation. Each line should contain one argument element or be left blank.
  k) LTAC is a code. In Markdown you should place 3 backticks ` ``` ` on a line above and below the code block to present LTAC arguments.

# LTAC example
```
- Claim G1: The system is acceptably safe for operation
  - Context X1: System specification (Dev17_specification_v1.3.pdf)
  - Strategy S1: Decompose safety argument by system hazards
    - Claim G2: All hazards have been identified
      - Evidence Ev1: Hazard analysis (HARA report.pdf)
    - Claim G3: All hazards have been mitigated
      - Link Ev1
```

# LTAC mapping to GSN (Goal Structuring Notation)
LTAC arguments can be mapped to GSN.
 - Claim in LTAC corresponds to Goal in GSN.
 - Evidence in LTAC corresponds to Solution in GSN.
 - The remaining element type names are consistent in LTAC and GSN.
 - Links in LTAC are used to cite elements in GSN which support more than one argument element.

# LTAC mapping to TRUST-IT
LTAC arguments can be mapped to TRUST-IT.
 - Justification in LTAC corresponds to Rationale in TRUST-IT.
 - Evidence in LTAC corresponds to Reference in TRUST-IT.
 - Context in LTAC corresponds to Information in TRUST-IT.
 - The remaining element type names are consistent in LTAC and TRUST-IT.
 - Links in LTAC are used to cite elements in TRUST-IT which support more than one argument element.
 
# LTAC mapping to CAE (Claims Arguments Evidence)
LTAC arguments can be mapped to CAE.
 - Strategy in LTAC corresponds to Argument in CAE.
 - Assumption in LTAC corresponds to Claim accepted as true in CAE.
 - Justification in LTAC corresponds to Side Claim in CAE.
 - The remaining element type names are consistent in LTAC and CAE.
 - Links in LTAC are used to cite elements in CAE which support more than one argument element.


# EBNF grammar for LTAC argument:

LTACargument    = ElementList ;

ElementList     = { Element } ;

Element         = Indent "-" WS NodeType [WS Identifier] ":" WS Text [WS Reference] Newline
                  [ ElementList ] 
                | Indent "-" WS "Link" WS Identifier Newline ;

NodeType        = "Claim" | "Strategy" | "Evidence" | "Justification" | "Context" | "Assumption";

Identifier      = { Character - ":" - Newline } ;   (* e.g., C1, C1.1, Ev5, J1, X3 *)

Text            = { Character - Newline } ;       (* descriptive label text *)

Reference       = "(" , { Character - ")" - Newline } , ")" ; (* e.g. (Doc-HAZ-2025) *)

Indent          = { "  " } ;   (* each indentation level = 2 spaces *)

Newline         = "\n" ;

WS              = " " | "\t" ;
Character       = ? any printable character ? ;