I am learning to use the Lean 4 proof assistant. I have a simple question about what the underscore symbol _ does in the following context:

The following example (src) works fine.

example {p q : ℚ} (h1 : p - 2 * q = 1) (h2 : q = -1) : p = -1 :=
  calc
    p = (p - 2 * q) + (2 * q) := by ring
    _ = (1) + (2 * -1) := by rw [h1,h2]
    _ = -1 := by ring

However, as a beginner, I’m tempted to use actual variable name p at the start of each line as follows:

example {p q : ℚ} (h1 : p - 2 * q = 1) (h2 : q = -1) : p = -1 :=
  calc
    p = (p - 2 * q) + (2 * q) := by ring
    p = (1) + (2 * -1) := by rw [h1,h2]
    p = -1 := by ring

gives the following error:

invalid 'calc' step, left-hand-side is
  p : ℚ
previous right-hand-side is
  p - 2 * q + 2 * q : ℚ

Q: why is this?

I would have expected p = to be valid and justified as the start of each proof line, and that _ = merely to be a shorthand for “previous” much like we write pen and paper proof where we omit the LHS for all but the first line.

Clearly _ is doing something I don’t understand or expect.