[CAS] PART FUNCTION on sum of n terms

[compsystems]

HP48 to HP50 also grouped the terms of addition and multiplication by twos

PHP Code:

« 0 0 { } -> EXPRESSION XPART NPARTS OPERATOR OBJECTS
  « EXPRESSION
    IFERR OBJ->
    THEN ->STR 'OPERATOR' STO 0 'NPARTS' STO OPERATOR 1 ->LIST 'OBJECTS' STO
    ELSE ->STR 'OPERATOR' STO 'NPARTS' STO NPARTS ->LIST 'OBJECTS' STO NPARTS OPERATOR OBJECTS 3 ->LIST DROP
    END
      IF XPART NPARTS <= XPART -1 >= AND NOT
      THEN "EXPRESSION " EXPRESSION + " CONTAINS ONLY " + NPARTS + " PARTS" +
      ELSE
        IF XPART 0 ==
        THEN OPERATOR
        ELSE
          IF XPART -1 ==
          THEN NPARTS
          ELSE OBJECTS XPART GET
          END
        END
      END
  »
»
'PART' STO 


'x^3-6*x^2+11*x-6' -1 PART @ returns 2 (parts)
'x^3-6*x^2+11*x-6' 0 PART @ returns "-"
'x^3-6*x^2+11*x-6' 1 PART @ returns '(x^3-6*x^2+11*x)'
'x^3-6*x^2+11*x-6' 2 PART @ returns 6
'x^3-6*x^2+11*x-6' 3 PART @ returns "EXPRESSION 'x^3-6*x^2+11*x-6' CONTAINS ONLY 2 PARTS"
'x^3-6*x^2+11*x-6' -3 PART @ returns "EXPRESSION 'x^3-6*x^2+11*x-6' CONTAINS ONLY 2 PARTS"

'X' -1 PART @ returns 0 (parts)
'X' OBJ->@ returns "ERROR" =(
'X^1' EVAL OBJ-> @ returns"ERROR" =(
'X^1' EVAL -1 PART @ returns 0 (parts)

'-X' -1 PART @ returns 1
'-X' 0 PART @ returns "NEG"
'-X' 1 PART @ returns 'X'

e -1 PART @ returns 0
e 0 PART @ returns "e"

pi -1 PART @ returns 0
pi 0 PART @ returns "pi"


'1/1 + -1/2 + 1/3 + -1/4 + 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + -1/10 ' -1 PART @ returns 2
'1/1 + -1/2 + 1/3 + -1/4 + 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + -1/10 ' 0 PART @ returns "+"
'1/1 + -1/2 + 1/3 + -1/4 + 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + -1/10 ' 1 PART @ '-1/10 '
'1/1 + -1/2 + 1/3 + -1/4 + 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + -1/10 ' 2 PART @ '1/1 + -1/2 + 1/3 + -1/4 + 1/5 + -1/6 + 1/7 + -1/8 + 1/9'