Context. A scores table is wide — one column per subject. For analysis you need it long: one row per student-subject pair.
Your task. Write to_long(df) that melts the wide table into columns student, subject, and score.
Example.
| student | math | science |
|---|---|---|
| Alice | 90 | 85 |
becomes rows (Alice, math, 90) and (Alice, science, 85).
Notes. student stays as an identifier column. Every other column becomes a subject value.