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: ::= A | B | turn() | sew(,) 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: A // /+ turn(A) \ +\ turn(turn(A)) +/ // turn(turn(turn(A))) + \ B -- -- turn(B) || || 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. Input 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. Output 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.

2/2 Sample Input 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)))))) ; Sample Output Quilt 1: ||-- ||-- --|| --|| Quilt 2: error Quilt 3: \// +/+ +/+ //\