Little Quilt

Little Quilt is a small language introduced by Ravi Sethi in his book ‘Programming Languages’. Here, a restricted version of Little Quilt is presented.

The language is defined by the following BNF grammar:

<QUILT> ::= A | B | turn(<QUILT>) | sew(<QUILT>,<QUILT>)

A and B represent the two primitive quilts. Each primitive quilt corresponds to a matricial arrangement of 2 × 2 characters. turn() and sew() are operations over quilts.

The instruction turn(x) turns the quilt x 90 degrees clockwise. The following table illustrates the primitive quilts as well as examples of the effect of the turn() operation:

Accordingly, the instruction sew(x,y) sews quilt x to the left of quilt y. Both x and y must have the same height, otherwise an error will be generated. The following figure represents the result of sew(A,turn(B)):

//||
/+||

while the sew(turn(sew(B,turn(B))),A) generates an error message.

Your job is to build an interpreter of the Little Quilt language.

The input file will be a text file containing different Little Quilt expressions, each one ended by a semicolon character (;). Space and new line characters must be ignored; this means that an expression may span several lines.

The output file contains the quilts produced as a result of interpreting the input expressions.

Each quilt must be preceded by a line, left aligned, with the format

Quilt i:

where i is the quilt number, starting at 1. If the expression interpretation generates and error, the word

error

must be printed.

sew(turn(sew(B,turn(B))),
       turn(sew(turn(B),B)))   ;


   sew(turn(sew(B,turn(B))),A);
sew(turn(sew(A,turn(A))),
 turn(turn(

   turn(sew(A,turn(A))))))

   ;

Quilt 1:
||--
||--
--||
--||
Quilt 2:
error
Quilt 3:
\\//
+\/+
+/\+
//\\