function parse(p) {
  setProgram(p);
  lex();
  let result = parseExpression();
   if (lookahead !== null)
    throw new SyntaxError(`Unexpected input after reached the end of parsing ${lineno}: ${program.slice(0,10)}`);
 
  return result;
}

function parseExpression() {
  let expr;
 
  if (lookahead.type == "STRING") {
    expr = {type: "value", value: lookahead.value};
    lex();
    return expr;
  } else if (lookahead.type == "NUMBER") {
    expr = {type: "value", value: lookahead.value};
    lex();
    return expr;
  } else if (lookahead.type == "WORD") {
    expr = {type: "word", name: lookahead.value};
    lex();
    return parseApply(expr);
  } else {
    throw new SyntaxError(`Unexpected syntax line ${lineno}: ${program.slice(0,10)}`);
  }
}

function parseApply(tree) {
  if (!lookahead) return tree;   // apply: /* vacio */
  if (lookahead.type !== "LP") return tree; // apply: /* vacio */
 
  lex();
 
  tree = {type: 'apply', operator: tree, args: []};
  while (lookahead && lookahead.type !== "RP") {
    let arg = parseExpression();
    tree.args.push(arg);
 
    if (lookahead && lookahead.type == "COMMA") {
      lex();
    } else if (!lookahead || lookahead.type !== "RP") {
      throw new SyntaxError(`Expected ',' or ')'  at line ${lineno}: ... ${program.slice(0,20)}`);
    }
  }
  if (!lookahead)  throw new SyntaxError(`Expected ')'  at line ${lineno}: ... ${program.slice(0,20)}`);
  lex();
 
  return parseApply(tree);
}