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Jason is a fully-fledged interpreter for an extended version of AgentSpeak, a BDI agent-oriented logic programming language.

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package jason.stdlib;

import jason.JasonException;
import jason.asSemantics.DefaultInternalAction;
import jason.asSemantics.InternalAction;
import jason.asSemantics.TransitionSystem;
import jason.asSemantics.Unifier;
import jason.asSyntax.ListTerm;
import jason.asSyntax.StringTerm;
import jason.asSyntax.StringTermImpl;
import jason.asSyntax.Term;

/**
  Internal action: .concat.

  
Description: concatenates strings or lists.

  
Parameters:

  + arg[0] ... + arg[n-1] (any term): the terms to be concatenated.

  
+/- arg[n]: the result of the concatenation.
  
  Parameters that are not string are concatenated using the toString method of
  their class.

  Examples:

   .concat("a","b",X): X unifies with "ab".
  
 .concat("a","b","a"): false.
  
 .concat("a b",1,a,X): X unifies with "a b1a".
  
 .concat("a", "b", "c", "d", X): X unifies with "abcd".
  
 .concat([a,b,c],[d,e],[f,g],X): X unifies with [a,b,c,d,e,f,g].
  

  Note: this internal action does not implement backtrack. You if need backtrack, you can add
  and use the following rules in your code:
  
  concat([ ], L, L).
  concat([H|T], L, [H|M]) :- concat(T, L, M).
  

  @see jason.stdlib.delete
  @see jason.stdlib.length
  @see jason.stdlib.member
  @see jason.stdlib.sort
  @see jason.stdlib.substring
  @see jason.stdlib.nth
  @see jason.stdlib.max
  @see jason.stdlib.min
  @see jason.stdlib.reverse

  @see jason.stdlib.difference
  @see jason.stdlib.intersection
  @see jason.stdlib.union

*/
public class concat extends DefaultInternalAction {

    private static InternalAction singleton = null;
    public static InternalAction create() {
        if (singleton == null)
            singleton = new concat();
        return singleton;
    }

    @Override
    public Object execute(TransitionSystem ts, Unifier un, Term[] args) throws Exception {

        if (args[0].isList()) {
            // list concat
            if (!args[args.length-1].isVar() && !args[args.length-1].isList()) {
                throw new JasonException("Last argument of concat '"+args[args.length-1]+"'is not a list nor a variable.");
            }
            ListTerm result = (ListTerm)args[0].clone();
            for (int i=1; i